The spec does not require Open API 3.x specification, but it's highly recommended to be provided by services.
So the client for OGC API Features should also support reading capabilities from Open API 3.x spec.
OGC API Features defines requirements (conformance) classes:
http://www.opengis.net/spec/ogcapi-features-1/1.0/req/oas30 => Servers conforming to this requirements class define their API by an OpenAPI Document
OGC API Features - Part 1 : Core describes support for API definitions.
The spec does not require Open API 3.x specification, but it's highly recommended to be provided by services.
So the client for OGC API Features should also support reading capabilities from Open API 3.x spec.
OGC API Features defines requirements (conformance) classes:
http://www.opengis.net/spec/ogcapi-features-1/1.0/req/oas30
=> Servers conforming to this requirements class define their API by an OpenAPI Document