Crank it up and let it idle. If the light goes out for a minute or two then comes back on, shut it down, wait about 1 beer, (not too long, don't want the engine to cool down) and start it again. If it happens all over again just the same, you probably have a bad oil pressure switch. If the diaphragm in the switch has a leak, it will work fine for a short time, then enough oil leaks to the other side to ballance the pressure and the light comes on. If this is the case, there'll be no harm to the engine, BUT it is a bad idea to ride it like that because there is no way to know if you really do lose pressure.
