WARNING: This page is about a different car, the 2003 Nissan Altima. However, it is still accessible from the selected car via links, so may be relevant.
Check continuity between the oil pressure switch and body ground.
Fig 1: Oil Pressure And Continuity
Courtesy of NISSAN MOTOR CO., U.S.A.
Fig 2: Checking Continuity Between Oil Pressure Switch And Body Ground