1. Key difference 1: Data query and manipulation capabilities: OData provides well-defined query and manipulation capabilities for data sources, allowing clients to perform complex operations like filtering, sorting, and paging directly on the server-side. On the other hand, OpenAPI does not include any specific query or manipulation capabilities for data sources.
2. Key difference 2: Protocol and format: OData is a protocol that supports multiple data formats such as JSON and Atom, providing a standardized way of interacting with data services over HTTP. OpenAPI, on the other hand, is a specification for defining RESTful APIs, focusing on documenting the operations, parameters, and responses of the API endpoints rather than specifying the protocol or format.
3. Key difference 3: Data model: OData has a built-in data model concept called the Entity Data Model (EDM), which allows for defining and representing structured data along with its relationships. OpenAPI does not have any built-in data model concept and only focuses on describing the operations and endpoints of an API.
4. Key difference 4: Hypermedia support: OData includes support for hypermedia controls, which provide navigation capabilities between related entities within the API. OpenAPI does not have native support for hypermedia controls and relies on the conventions or extensions to achieve similar functionality.
5. Key difference 5: Client and server implementations: OData specifies a set of guidelines and conventions to enable the implementation of both client and server components that adhere to the protocol. OpenAPI, on the other hand, only focuses on documenting the API and does not provide any guidelines or conventions for implementing client or server components.
6. Key difference 6: Extensibility: OData provides extensibility options like custom query options, annotations, and extensions to extend the protocol beyond its standard capabilities. OpenAPI, on the other hand, does not have any native extensibility mechanisms and relies on additional specifications or conventions to add custom functionality to the API.

In Summary, OData and OpenAPI differ in terms of data query and manipulation capabilities, protocol and format, data model, hypermedia support, client and server implementations, and extensibility options.