diff --git a/CHANGELOG.md b/CHANGELOG.md
index d520d3f8c..3d4e347b0 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -60,6 +60,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 - Replaced the deprecated fs.rmdirSync with fs.rmSync ([#359](https://github.com/opensearch-project/opensearch-api-specification/pull/359))
 - Tester tool now provides better context for non-2XX responses when --verbose is used ([#359](https://github.com/opensearch-project/opensearch-api-specification/pull/359))
 - Lock testing for next release of OpenSearch to a specific SHA ([#431](https://github.com/opensearch-project/opensearch-api-specification/pull/431))
+- Replace nullable with null type ([#436](https://github.com/opensearch-project/opensearch-api-specification/pull/436))
 
 ### Deprecated
 
diff --git a/spec/schemas/_common.aggregations.yaml b/spec/schemas/_common.aggregations.yaml
index f2c25a65f..3313b096b 100644
--- a/spec/schemas/_common.aggregations.yaml
+++ b/spec/schemas/_common.aggregations.yaml
@@ -113,21 +113,14 @@ components:
     KeyedPercentiles:
       type: object
       additionalProperties:
-        oneOf:
-          - type: string
-          - type: number
-          - nullable: true
-            type: string
+        type: ['null', number, string]
     ArrayPercentilesItem:
       type: object
       properties:
         key:
           type: string
         value:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         value_as_string:
           type: string
       required:
@@ -162,10 +155,7 @@ components:
               description: |-
                 The metric value. A missing value generally means that there was no data to aggregate,
                 unless specified otherwise.
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             value_as_string:
               type: string
           required:
@@ -226,20 +216,11 @@ components:
             count:
               type: number
             min:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             max:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             avg:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             sum:
               type: number
             min_as_string:
@@ -266,40 +247,19 @@ components:
         - type: object
           properties:
             sum_of_squares:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             variance:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             variance_population:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             variance_sampling:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             std_deviation:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             std_deviation_population:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             std_deviation_sampling:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             std_deviation_bounds:
               $ref: '#/components/schemas/StandardDeviationBounds'
             sum_of_squares_as_string:
@@ -326,35 +286,17 @@ components:
       type: object
       properties:
         upper:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         lower:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         upper_population:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         lower_population:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         upper_sampling:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         lower_sampling:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
       required:
         - lower
         - lower_population
@@ -1426,32 +1368,20 @@ components:
             count:
               type: number
             min_length:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             max_length:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             avg_length:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             entropy:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             distribution:
               oneOf:
+                - type: 'null'
                 - type: object
                   additionalProperties:
                     type: number
-                - nullable: true
-                  type: string
+                - type: string
             min_length_as_string:
               type: string
             max_length_as_string:
@@ -1522,17 +1452,11 @@ components:
         sort:
           type: array
           items:
-            oneOf:
-              - $ref: '_common.yaml#/components/schemas/FieldValue'
-              - nullable: true
-                type: string
+            $ref: '_common.yaml#/components/schemas/FieldValue'
         metrics:
           type: object
           additionalProperties:
-            oneOf:
-              - $ref: '_common.yaml#/components/schemas/FieldValue'
-              - nullable: true
-                type: string
+            $ref: '_common.yaml#/components/schemas/FieldValue'
       required:
         - metrics
         - sort
@@ -1542,10 +1466,7 @@ components:
         - type: object
           properties:
             value:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             value_as_string:
               type: string
           required:
@@ -2544,21 +2465,13 @@ components:
       properties:
         from:
           description: Start of the range (inclusive).
-          oneOf:
-            - type: number
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         key:
           description: Custom key to return the range with.
           type: string
         to:
           description: End of the range (exclusive).
-          oneOf:
-            - type: number
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', number, string]
     GeoHashGridAggregation:
       allOf:
         - $ref: '#/components/schemas/BucketAggregationBase'
@@ -2726,19 +2639,13 @@ components:
       properties:
         from:
           description: Start of the range.
-          oneOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         mask:
           description: IP range defined as a CIDR mask.
           type: string
         to:
           description: End of the range.
-          oneOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
     IpPrefixAggregation:
       allOf:
         - $ref: '#/components/schemas/BucketAggregationBase'
@@ -3113,8 +3020,8 @@ components:
                 - type: array
                   items:
                     type: number
-                - nullable: true
-                  type: string
+                - type: 'null'
+                - type: string
             hdr:
               $ref: '#/components/schemas/HdrMethod'
             tdigest:
diff --git a/spec/schemas/_common.query_dsl.yaml b/spec/schemas/_common.query_dsl.yaml
index 3fab6cef8..df09be4fd 100644
--- a/spec/schemas/_common.query_dsl.yaml
+++ b/spec/schemas/_common.query_dsl.yaml
@@ -1417,13 +1417,11 @@ components:
             from:
               oneOf:
                 - $ref: '_common.yaml#/components/schemas/DateMath'
-                - nullable: true
-                  type: string
+                - type: 'null'
             to:
               oneOf:
                 - $ref: '_common.yaml#/components/schemas/DateMath'
-                - nullable: true
-                  type: string
+                - type: 'null'
             format:
               $ref: '_common.yaml#/components/schemas/DateFormat'
             time_zone:
@@ -1459,15 +1457,9 @@ components:
               description: Less than or equal to.
               type: number
             from:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
             to:
-              oneOf:
-                - type: number
-                - nullable: true
-                  type: string
+              type: ['null', number, string]
     RankFeatureQuery:
       allOf:
         - $ref: '#/components/schemas/QueryBase'
diff --git a/spec/schemas/_common.yaml b/spec/schemas/_common.yaml
index b080de313..365c8826d 100644
--- a/spec/schemas/_common.yaml
+++ b/spec/schemas/_common.yaml
@@ -146,14 +146,12 @@ components:
       type: string
     FieldValue:
       description: A field value.
-      oneOf:
-        - type: number
-        - type: number
-        - type: string
-        - type: boolean
-        - nullable: true
-          type: string
-        - type: object
+      type: 
+        - boolean
+        - 'null'
+        - number
+        - object
+        - string
     Void:
       description: |-
         The absence of any type. This is commonly used in APIs that don't return a body.
@@ -1282,8 +1280,7 @@ components:
         licensed:
           type: boolean
         custom_foldername:
-          nullable: true
-          type: string
+          type: ['null', string]
         opensearch_version:
           $ref: '#/components/schemas/VersionString'
       required:
@@ -2014,8 +2011,7 @@ components:
         relocating_node:
           oneOf:
             - $ref: '#/components/schemas/NodeId'
-            - nullable: true
-              type: string
+            - type: 'null'
         relocation_failure_info:
           $ref: '#/components/schemas/RelocationFailureInfo'
       required:
diff --git a/spec/schemas/_core.bulk.yaml b/spec/schemas/_core.bulk.yaml
index a4e03fd19..9006cfe23 100644
--- a/spec/schemas/_core.bulk.yaml
+++ b/spec/schemas/_core.bulk.yaml
@@ -116,10 +116,7 @@ components:
           x-version-removed: '2.0'
         _id:
           description: The document ID associated with the operation.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         _index:
           description: |-
             Name of the index associated with the operation.
diff --git a/spec/schemas/_core.rank_eval.yaml b/spec/schemas/_core.rank_eval.yaml
index 171f44789..4527f0c05 100644
--- a/spec/schemas/_core.rank_eval.yaml
+++ b/spec/schemas/_core.rank_eval.yaml
@@ -156,10 +156,7 @@ components:
         hit:
           $ref: '#/components/schemas/RankEvalHit'
         rating:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
       required:
         - hit
     RankEvalHit:
diff --git a/spec/schemas/_core.search.yaml b/spec/schemas/_core.search.yaml
index c6e65d005..87a2e01ef 100644
--- a/spec/schemas/_core.search.yaml
+++ b/spec/schemas/_core.search.yaml
@@ -19,10 +19,7 @@ components:
           items:
             $ref: '#/components/schemas/Hit'
         max_score:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
       required:
         - hits
     TotalHits:
@@ -50,10 +47,7 @@ components:
         _id:
           $ref: '_common.yaml#/components/schemas/Id'
         _score:
-          oneOf:
-            - type: number
-            - nullable: true
-              type: string
+          type: ['null', number, string]
         _explanation:
           $ref: '_core.explain.yaml#/components/schemas/Explanation'
         fields:
diff --git a/spec/schemas/cat.allocation.yaml b/spec/schemas/cat.allocation.yaml
index cfb016e91..ba6c82026 100644
--- a/spec/schemas/cat.allocation.yaml
+++ b/spec/schemas/cat.allocation.yaml
@@ -18,8 +18,7 @@ components:
             IMPORTANT: This metric double-counts disk space for hard-linked files, such as those created when shrinking, splitting, or cloning an index.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/ByteSize'
-            - nullable: true
-              type: string
+            - type: 'null'
         disk.used:
           description: |-
             Total disk space in use.
@@ -28,8 +27,7 @@ components:
             Unlike `disk.indices`, this metric does not double-count disk space for hard-linked files.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/ByteSize'
-            - nullable: true
-              type: string
+            - type: 'null'
         disk.avail:
           description: |-
             Free disk space available to OpenSearch.
@@ -37,32 +35,27 @@ components:
             Disk-based shard allocation uses this metric to assign shards to nodes based on available disk space.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/ByteSize'
-            - nullable: true
-              type: string
+            - type: 'null'
         disk.total:
           description: Total disk space for the node, including in-use and available space.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/ByteSize'
-            - nullable: true
-              type: string
+            - type: 'null'
         disk.percent:
           description: Total percentage of disk space in use. Calculated as `disk.used / disk.total`.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/Percentage'
-            - nullable: true
-              type: string
+            - type: 'null'
         host:
           description: Network host for the node. Set using the `network.host` setting.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/Host'
-            - nullable: true
-              type: string
+            - type: 'null'
         ip:
           description: IP address and port for the node.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/Ip'
-            - nullable: true
-              type: string
+            - type: 'null'
         node:
           description: Name for the node. Set using the `node.name` setting.
           type: string
diff --git a/spec/schemas/cat.indices.yaml b/spec/schemas/cat.indices.yaml
index 974f4cbfc..32e50795c 100644
--- a/spec/schemas/cat.indices.yaml
+++ b/spec/schemas/cat.indices.yaml
@@ -29,16 +29,10 @@ components:
           type: string
         docs.count:
           description: available docs
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         docs.deleted:
           description: deleted docs
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         creation.date:
           description: index creation date (millisecond value)
           type: string
@@ -47,16 +41,10 @@ components:
           type: string
         store.size:
           description: store size of primaries & replicas
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         pri.store.size:
           description: store size of primaries
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         completion.size:
           description: size of completion
           type: string
diff --git a/spec/schemas/cat.shards.yaml b/spec/schemas/cat.shards.yaml
index 65c9e94d2..4bb492c13 100644
--- a/spec/schemas/cat.shards.yaml
+++ b/spec/schemas/cat.shards.yaml
@@ -29,31 +29,19 @@ components:
           type: string
         docs:
           description: The number of documents in the shard.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         store:
           description: The disk space used by the shard.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         ip:
           description: The IP address of the node.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         id:
           description: The unique identifier for the node.
           type: string
         node:
           description: The name of node.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         sync_id:
           description: The sync identifier.
           type: string
diff --git a/spec/schemas/cat.templates.yaml b/spec/schemas/cat.templates.yaml
index 81c048955..ebc269250 100644
--- a/spec/schemas/cat.templates.yaml
+++ b/spec/schemas/cat.templates.yaml
@@ -21,8 +21,7 @@ components:
           description: The template version.
           anyOf:
             - $ref: '_common.yaml#/components/schemas/VersionString'
-            - nullable: true
-              type: string
+            - type: 'null'
         composed_of:
           description: The component templates that comprise the index template.
           type: string
diff --git a/spec/schemas/cat.thread_pool.yaml b/spec/schemas/cat.thread_pool.yaml
index bc91a875e..3d14b730a 100644
--- a/spec/schemas/cat.thread_pool.yaml
+++ b/spec/schemas/cat.thread_pool.yaml
@@ -60,25 +60,13 @@ components:
           type: string
         core:
           description: The core number of active threads allowed in a scaling thread pool.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         max:
           description: The maximum number of active threads allowed in a scaling thread pool.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         size:
           description: The number of active threads allowed in a fixed thread pool.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         keep_alive:
           description: The thread keep alive time.
-          anyOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
diff --git a/spec/schemas/indices.stats.yaml b/spec/schemas/indices.stats.yaml
index 45076740f..d3301fcd0 100644
--- a/spec/schemas/indices.stats.yaml
+++ b/spec/schemas/indices.stats.yaml
@@ -255,10 +255,7 @@ components:
         primary:
           type: boolean
         relocating_node:
-          oneOf:
-            - type: string
-            - nullable: true
-              type: string
+          type: ['null', string]
         state:
           $ref: '#/components/schemas/ShardRoutingState'
       required:
diff --git a/tools/src/_utils/JsonSchemaValidator.ts b/tools/src/_utils/JsonSchemaValidator.ts
index e72b5654b..ff33dbc1f 100644
--- a/tools/src/_utils/JsonSchemaValidator.ts
+++ b/tools/src/_utils/JsonSchemaValidator.ts
@@ -22,7 +22,8 @@ interface JsonSchemaValidatorOpts {
 
 const DEFAULT_AJV_OPTS = {
   strict: true,
-  allErrors: true
+  allErrors: true,
+  allowUnionTypes: true
 }
 
 // Wrapper for AJV
@@ -58,4 +59,4 @@ export default class JsonSchemaValidator {
     const errors = is_schema ? this.ajv.errors : validate_func.errors
     return valid ? undefined : this.errors_parser.parse(errors)
   }
-}
\ No newline at end of file
+}