Good day, I need some direction here on what the hell is going on with this S60 T5,

firstly all the pressure tests have been done and there is no leaks on the outside, there's no oil mixing with water, no smoke nothing the cambelt has been done and the car only has 99 000km on it. so here are the two issues,
the coolant light keeps coming on and there is oil on the plugs, any advise please guys?

 

If you have pressure tested the cooling system, and there are no external leaks and it holds pressure. - You could use a combustion gas detector on the cooling system to look for any leaks past the head gasket. That takes a sample of the air in the coolant bottle and if HCs are present changes the color of the test fluid.

Oil on the plugs? As in Liquid oil on the threads when you removed them? If so the cam cover is leaking slightly into the spark plug wells. Not usually anything to worry about - have seen it many times during spark plug replacement.