Guttooth71 - A few questions to help us understand better:

By saying that the bike lost power and the oil light came on...do you mean that the engine shut down completely and you coasted to the side of the road? If so, with the ignition on and the engine stopped, the oil light being on would be expected...the oil pressure sensor would see no oil pressure because the engine has stopped. Then it just becomes a matter of determining what shut her down (sidestand switch, kill switch, ignition switch, etc). Could be quite simple.

IF instead you mean that the engine was "down on power" but still running as you stopped with the oil light on, then I think we're looking at something quite different. There, I'd begin to suspect a bad oil pressure sensor, sensor wiring or oil pump (although I've never heard of a GL1800 failing an oil pump).

When you say it would not start once you got to the side of the road...when you engaged the starter, did anything happen? Did the starter engage but was unable to turn the motor? Did the motor spin-spin-spin with no start?

Just trying to clarify the scenario details a bit so we can understand and (hopefully) help more.

Cheers!