First of all you schould check via the oil-check-window, if there is enough AND not too much oil in the oiltank (there are minimum and maximum lines).You schould check this with a slightly warmed up engine and the bike schould stay upright (maybe help from another person needed).
If this is ok. I would recommend to change the oil-pressure sensor.
If the light is still on, then there is something wrong with your engine.