If the oil warning light is coming on when the engine is hot and at low revs then it's the oil pressure that's low and not the level, so make sure the level is right, also the engine isn't overheating, the fan should come on if you're stationary and the coolant temp goes up, and the correct grade oil was used, 10w 40 .
If the above are correct then there are two possibilities, faulty oil pressure switch should think they're about £10 to £20 or worn bearings/ oil pump. don't ask?
how many miles has it done?