The standard (for me) OAuth flow is:

generate url flow.step1_get_authorize_url() and ask user to allow app access
get the code
get the credentials with flow.step2_exchange(auth_code)

But I faced with another service, where I just need to initiate a POST request to token_uri with client_id and client_secret passed as form values (application/x-www-form-urlencoded), grant_type is client_credentials and scope is also passed as form field value.
Does oauth2client library supports it?
Update the grant_type to implicit
How to get OAuth token without user consent?