1999 Vauxhall Corsa oil light comes on when hot

First thing to do would find and eliminate coolant loss .Check rad fan operating o.k. If no leaks apparent a pressure test on cooling system should show any leaks .Once leak fixed and if oil light still coming on , could be oil pressure switch or general wear and tear on engine causing oil pressure drop ..

If , after fixing coolant loss and no engine rattles , changing oil and filter might help also especially if not been done for a while .

Common fault on those engines is oil switch failure, I have changed many in the past but usually they leak oil every where. Has the oil been changed recently, maybe the oil is a bit thin, you may get an oil pressure warning at tickover, especially if the car has done a lot of miles,
as above you need to find out why or if the car is deffinatly overheating too.
In my opinion if the car is overheating it quite a lenghty job on a timing chain model which I would advise replacing at the same time(chain assembly that is) and if you have an oil pump issue too it could just be your engine is not worth it, money wise
, and better to fit a good low milage unit, this could be all easy cheap fixes but this is the worst case scenario, I've been there and done it, tried to repair multiple engine problems then had to change engine.