When the oil light is on, it is notifying you that there is not enough oil pressure in the engine to protect it. DO NOT RUN THE ENGINE IN THIS CONDITION. Doing so can cause permanent engine damage very quickly. Verify that your engine is not low on oil, that is the most common cause of no oil pressure.
You can test the circuit from the oil pressure switch to the light in the dash without running the engine. With the ignition key to the "on" position (but the engine not running), the oil light should be on. If you disconnect the wire from the oil pressure switch, the light should go off. If it does not, the wire is grounded somewhere between the switch and the light in the dash. If it works as described, the switch is telling you that there is no oil pressure. You can remove the switch and connect a pressure gauge to verify whether or not there is oil pressure, or simply replace the switch if you think it is faulty (should be around $10 for a new one).
If there is no oil pressure, you may have a clogged oil pickup in the oil pan or something wrong with the oil pump itself. The oil pump is replaceable without pulling the engine, but it may not be something you want to try yourself. You will need to remove the timing belt and the oil pan to get to the pump and the oil pickup, if you have to go that far you probably should replace the timing belt and the water pump while you have it apart.