You can define top-level parameters which can be references with $ref="#/components/parameters/$parameter"
        You can define top-level responses which can be references with $ref="#/components/responses/$response"

    I find it usefull to add @OA\Response(ref="#/components/responses/todo") to the operations when i'm starting out with writting the swagger documentation.
    As it bypasses the "@OA\Get() requires at least one @OA\Response()" error and you'll get a nice list of the available api calls in swagger-ui.

    Then later, a search for '#/components/responses/todo' will reveal the operations I haven't documented yet.
    
    And although definitions are generally used for model-level schema's' they can be used for smaller things as well.
    Like a @OA\Schema, @OA\Property or @OA\Items that is uses multiple times.