Normally when the oil warning light comes on � the oil pressure is so low, it is highly likely that the engine will seize (or significant wear occur). Best get it sorted ASAP, for not a lot of money your local garage should be able to check the pressure.
Check things as Parsfan advises � I have heard of a partial blockage (close to the oil pressure switch) causing a venturi effect, resulting in a reduced pressure reading (at the pressure switch), but otherwise the system oil pressure was OK.