No oil pressure

My 91 jimmy has been running fine, minor things here and there but a few days ago, the oil pressure gauge was registering a psi rating of about 30 to 40, and then all of a sudden it says i have no oil pressure. I talked to a buddy and he says its probably the oil pump. Can any one tell me for sure?

Always check oil pressure with a manual test gauge.
The if no pressure with a test gauge then check oil pump and oil pump pickup screen and engine bearings.