so I would replace the sending unit or screw in an actual oil pressure gauge, and if it still does it consider that indeed you have an oil supply or bearing problem. If the engine is worn, you can also go to a slightly thicker oil, for example if using 10w-30 then use 20w-50. Make sure the engine is clean with no debris blocking the cooling. Also if it comes on say after mowing 2 hours, give it a rest once in awhile, especially hot days. If the light is on ALL the time try removing the wire on the sender and see if it stops, if it does then replace the sender and see if that gets it. If the engine is in warranty then take it to the dealer. Definately be looking for metal debris in the oil or oil filter.