Oil pressure light keeps going on & off..

Have your mechanic check the actual oil pressure with a mechanical gauge. The pressure sending unit is the more common cause of the problem and some mechanics will just replace it without performing any tests first because the part costs less than the labor time to do a full diagnosis. He will guess right more often than not and save most of his customers some money.

When the most likely suspect doesn't solve the problem, he has to look further. Oil pumps rarely wear out to the point they can't develop any pressure, so unless he knows about some common failure, the next suspect is the engine bearings. There's no real easy way to diagnose that without replacing them, and that's pretty involved, so he probably wanted to replace the pump first to rule that out. The pump has a pressure-relief valve too that can get stuck open with debris and bleed off most of the pressure but that isn't very common.