Engine oil light is showing on the dash board after switching on

Most important thing you need to do is to establish the actual oil pressure. put a mechanical gauge on the engine and read real oil pressure. If pressure is low (min is 15psi, I prefer to see at least 20 at idle) there is an internal problem...bearings etc. If pressure is good, sending unit has failed...replace it. I would not drive it 'till the oil pressure is verified as being safe. You might get lucky and someone just forgot to plug in the sending unit, but it must be checked!!!
good luck

set the timing mark of the timing chain . even though its been replace you still have to set the timing mark of the timing chain for the engine to run good . May be that is the reason why dash board light is still on while running , don' t worry about the dash board its not grounded.