Hi, there are two oil pressure switches on the engine and if either of the switches goes faulty it should operate the light and buzzer. If you have checked the oil pressure with a guage and it is within specs, go for the switches. One of the switches is normaly open contacts and the other is normally closed. To test these I use 2 test lamps, one end of each connected to battery positive and the other end of the test lamp to each switch, then with engine stood one lamp should be lit, and when you start the engine it should go out and the other should light. Watch for either of them flickering or not changing over. If you do replace the switches, make sure that you put the correct switch in the correct location. I have also seen some cars with the strainer on the oil pick up pipe blocked, this causes the oil pressure to drop as the engine revs are increased, drop the sump off and clean out.
Steve.
