The best guess would be the printed circuit went bad when you had the dash out. Unplugging and plugging back in the dash harness from the back of the cluster could have been enough to ruin an aged printed circuit. One other thing, fumes from lacquer thinner can ruin an aged printed circuit also. For a test, you could test the oil pressure sender for the correct resistance. I don't have the exact readings you should get when testing. Someone may chime in to give you those details... Since this happened after you pulled the dash, checking the sending unit would just be a formality because it would have to be a coincidence that it went out in this course of events.
