Fastest way to get current user's OID in Postgres?

Question

I am thinking of storing the user's OID in the "owner" column of a table, so I don't have to do anything if they change their username.

I understand one can use pg_has_role() with OIDs, so that's good.

Is there a fast and easy way to get the current user's OID without just matching it against one of the system tables?