Oil light goes on and off just started happening

This is not unusual in old engines. The oil pressure drops when revs drop to idle speed, due to wear in the main and big end bearings. It isn't serious as long as the pressure rises with revs.

Once past the obvious - making sure your oil is the right grade and the level is correct - you could ask a mechanic to connect an oil pressure gauge to the oil pressure sensor port, and read the pressure at various revs. You should have about 10 psi pressure per 1000 revs.

Occasionally the oil pressure sensor malfunctions and reports a falsely low pressure. Also a possibility is a weak over pressure valve in the oil filter.
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Put oil in the engine