The oil pressure gauge sits at 0 and does not show any pressure when idling.
I'm guessing that you checked the oil level and it's OK. If so, most likely, it's the oil pump and/or pressure relief valve, or could be worn-out crankshaft and bearings. The oil pump runs at a fixed pressure and the relief valve opens to reduce it to operating pressure, then the bearings create pressure resistance. The Oil light is just alerting you that you have something seriously out of acceptable operating range. A more remote possibility is that the oil sending unit is bad.