The light goes on and off. sometimes staying on for 1/2 hour or so. the oil shows full. what could be going on?

Probably just a bad sending unit. Might be more but I'd start there by replacing it. Also, check the connection to it.

If the problem does not go away by replacing the sending unit, take the car in to a professional. They will need to remove the sending unit and temporarily replace it with a manual pressure gage and take a reading of the actual oil pressure. They will do this at various RPM's to determine if the engine is getting proper lubrication. Do this right away, since if you are driving the car around with low oil pressure, the engine will be destroyed rather quickly. It might just be a bad oil pump and replacing the oil pump could save the engine. If you drive the car to long with low oil pressure from a bad oil pump you, will need to replace the engine or decide whether the car is worth putting another engine in it. If it is low oil pressure from worn engine bearings, the engine will need to be replaced.

If the professional determines the engine is damaged due to engine sludge, do a search on this forum for "engine sludge" and read up on the problem. Toyota has extended the warranty to the generation 4 Camrys (1997-2001) due to engine sludging or oil gelling, as Toyota refers to it.

Mike