It could be that the sender is bad. Try buying a new sender at a parts store and install it. It may or may not fix the problem. Other things: dirty oil filter, oil return passages may be dirty, worn out engine, low oil levels, weak oil pump, cam bearings that drive the oil pump could be bad, and I'm sure there could be others as well. This is an indication that your engine may not be lubricating correctly. If you have a mechanical guage handy, screw it into the oil pressure sender (I believe that these are just switches and if the switch goes out, it will send a no oil pressure signal). I've heard this can be pretty common on most cars and should be your first step to diagnosing your low oil pressure levels.