When an HTTP request is received and the HTTP Caching policy is applied first, it checks if there is a cached response for that request in Object Store. If there is a cache hit, meaning that a valid cached response exists, then only the HTTP Caching policy is evaluated and the cached response is returned to the caller without invoking the OAuth policy or the API implementation. If there is a cache miss, meaning that no valid cached response exists, then both the HTTP Caching policy and the OAuth policy are evaluated before invoking the API implementation.
References:https://docs.mulesoft.com/api-manager/2.x/http-caching-policy#policy-ordering