Low Oil Pressure Warning Switch
WARNING: This page is about a different car, the 2002 Toyota Avalon. However, it is still accessible from the selected car via links, so may be relevant.
Disconnect single-pin Gray electrical harness connector from low oil pressure warning switch on engine. Using an ohmmeter, check that continuity exists between low oil pressure warning switch terminal and good engine ground point with engine off and that continuity does not exist with engine running. If continuity is not as specified, replace low oil pressure switch.