I was getting access denied when trying to get a Bearer token using OAuth Setup.
The missing piece for me was to set grant_type=client_credentials.
Here is a curl command:
curl -d "grant_type=client_credentials&client_id=[]&client_secret=[]&username=[]&password=[]" -X POST "https://[].service-now.com/oauth_token.do"
And postman screenshot:
