OneOf is a keyword defined by OpenAPI 3.0 that combines schemas to define a schema that can be validated against multiple criteria, making it easier for developers to define schemas and validate data. It ensures that the incoming value in a request or response should be valid against only one of the schemas defined by the API specification. OneOf can be used at different levels, including custom vs primitive types, field vs parameter, maps, arrays, and in both request and response types, and it supports use cases such as sending data to an endpoint that must conform to a specific format or validating data against multiple schemas. The keyword is also supported in various programming languages, including Java, through developer experience platforms like APIMatic.