Could be that you are low on oil or are using an oil that is too thin.
Beyond that, it could be a bad oil pump, worn crankshaft bearings, or a
defective oil pressure sensing switch. How many miles do you have on that
car? If it is beyond 200,000 - then my guess is that you need a new oil
pump. That is more likely than worn bearings.

Anyway, don't drive the car until you get it fixed. Otherwise you are
risking a blown engine. A new oil pump is cheap by comparison.