Why would oil pressure light come on when car is idling in park or in drive??

You need to have a shop check the oil level and pressure as a mechanical oil pump has low pressure at idle then as the motor revs if gains pressure. If the screen on the oil pump is beginning to clog or the oil pump itself is beginning to fail it can do major internal engine damage if its not addressed, You can find a local shop here www.repairpal.com/directory