OperationOutcome could be used as a response body for any HTTP 4xx or 5xx response on the REST API [http://www.hl7.org/fhir/http.html#2.1.0.4].
I can see that this profile is intended to be used for specific interactions (GP Connect Query request). The name would be better if it were more specific. The profile is overly restrictive - if an implementation framework detects another mode of failure (e.g. 401 Unauthorized, 403 Forbidden, 500 server error) and can transform this to a standard OperationOutcome then it would be better to be able to provide an informative response body