I had the same problem after having the engine out of the bike for a few months, totally dry of oil. I found that just using the electric starter, or by kicking it over wasn't enough to get the oil pump working. What I did was remove the tappet adjuster caps, and squirt oil all over the cam and rockers to avoid starting a "dry" valve train, replaced the caps and started the engine. After only 4 or 5 seconds or running, the oil light went out. Just don't "rev" it and let it idle. I guess the oil pump need some real speed to get pumping properly. That worked for me, your results may vary.