What causes the Check Oil light to come on and the oil pressure to drop when the engine warms up on a 1993 Pontiac Grand Am?

If the main and/or rod bearings are worn, the oil pressure can't be maintained after the oil warms up and thins out. You might be able to get a little more life out of the engine by switching to a heavier weight engine oil.

It is possible also that just the oil pump has worn and can't produce enough pressure at idle. Very often, though, if the pump is worn, so are the bearings, and vice-versa.