In Tableau’s Databricks connector, OAuth is user-based (U2M) — it always logs in as a person via SSO. It does not support logging in as a service principal through the interactive OAuth flow.
If you need a non-human identity (e.g., for scheduled refresh or to avoid dependency on a user account), you must use a PAT generated for the service principal/Headless (or a service principal–based token). OAuth in Tableau is not designed for SP login.