oil warning light

[2kBoxster]

Having some problems with my 2000 Boxster and the mechanics do not know what is wrong.
Oil Warning Light was coming on once in awhile, but when you accelerated or raised the RPM's the light would go off. So figured it was just low on oil, added 1 Quart and then I checked the level and it looked full. But the light returned and now the Engine Warning Light is coming on once in awhile. Took it to mechanic and they heard a whining noise when we started the car up. They said serpentine belt had to be replaced. Took engine apart and belt was fine, and with belt off, it still makes whining noise. They are now talking about oil pressure and oil pressure canister, but they said they will have to tear engine apart to try to find out what is wrong.
Note: When they hooked the car up to the machine to read the CEL, it said it was the Oxygen Sensor.
Does anybody have some thoughts on this or what might be wrong?