a null value for oauth_application_id usually means the observation was created via the web, but it doesn’t completely rule out other means of creation.
that said, depending on what you’re trying to do, you could carve out a portion of records returned from the API by parsing through the oauth_application_id values returned with each observation record. or if you have more observations (up to 200000) in that larger set to be carved, you could do something similar to the 3rd paragraph here: https://forum.inaturalist.org/t/selectively-bulk-update-time-of-observation-based-on-the-camera-used/42571.