I learned the followings can turn the oil pressure light and buzzer ON:

- wrong oil grade
- wrong oil filter
- too much oil (level over or too close to the top mark on dipstick)
- too little oil (under the bottom mark on dipstick)
- weak oil pressure relief spring
- worn oil pressure relief valve
- faulty or shorted oil pressure sensors
- worn oil pump

I had the oil pressure light and buzzer symptom too... I did
- new oil pressure relief valve and spring
- new oil pressure sensors
- put oil under the top mark on dipstick (2mm under)
- install Vanistan oil cooler with fan