You definitely have some kind of problem there.... hopefully just electrical but possibly also an oil pressure problem.
Obvious reasons for an electrical problem - bad battery, bad alternator, bad ground somewhere.. Check for loose battery terminals, and also the alternator ground.
When voltage is low, some gages go crazy - does the speedo fluctuate a little when the car is stopped and idling?
Your oil pressure reading is in PSI btw. According to GM, it should be around 60PSI @ 2000 RPM. Many high-milage 3800'* run with very low oil pressure, and lots of times that'* what kills them.
Did those two problems start at the same time? if they did, it'd be a good sign that it'* just an electrical issue. Actually it looks like your alternator isn't charging at all and you're running on the battery alone. For how long has it been running like that?