Is it OK to use motor oil as bar oil in a chainsaw?
It might be OK for a bit but I probably wouldn't do it. You could look up your owner's manual to see if they say anything. If you must use the chainsaw and have nothing else on hand, surely motor oil is better than nothing. I think bar oil is stickier than regular motor oil to prevent splattering as much.

FWIW I like to buy the "biodegradable" chain oil, since it invariably ends up all over the place.