Its could beither Low oil level, Low oil pressure, faulty oil pump mechanism, clogged filter, blocked oil passages or the oil pressure sending unit that triggers the warning light (or operates your gauge) has failed. On some late model vehicles, the warning light will come on (or a message will appear) if a sensor in the oil pan detects a low oil level. The light is supposed to come on before the level gets low enough to cause any damage. Either way, do NOT drive the vehicle.
Turn off engine, allow it it to sit for a while and then check oil level. If its low or hardly visible, the engine's either using it or Loosing it. Check leaks, blown gaskets and seals, usually around, sump, valve cover areas. If there's no leaks or drops and the engine isn't using it up either, than check oil presuure by removing the oil pressure sensor switch and installing a pressure guage. A rough figure of 70-85 psi us what you should be looking at.