I have a stone stock and never molested (to my knowledge) '77 Cutlass Supreme with the Olds 350R and the idiot lights. The oil pressure light in the gauge cluster will not illuminate-not during the bulb check sequence on startup, not if I pull the wire off the sender and ground it-nothing. I have swapped out the bulb-and-socket for the light with a known good unit-nothing. Aside from a broken wire or a bad printed circuit board on the cluster (which looks OK visually), is there anything else that I am missing that is a common bitchkitty on these cars? Electrically, it's a pretty smple circuit...

 

I'm assuming that the other lights are performing normally. If so, the B+ supply through the fuse panel must be O.K. You seem to be on the right trail. It is a sad connection somewhere probably. It could be at the bulb socket, in the printed circuit, in a wire, or in the instrument cluster connector at the firewall. Older vehicles sometime develop a bit of corrosion at connectors especially if they have outside storage. You could use a volt meter and trace the circuit to see where the voltage is lost. I don't know of any particular shortfall in the system. Perhaps someone else can contribute.

 

As Ozzie stated above check for B+ on the other side of the lamp. If you have B+, grounding the wire should have lit the lamp. With the wire disconnected from the sender and the key on, work your way from the sender to the lamp looking for 12v with a testlight or voltmeter. From the point that you see 12v, to the sender there is an open.