When i start the moter the oil gage goe...

The oil pressure sending unit has gone bad and needs to be replaced. There not that expensive and you can pick it up at Auto Zone, they can also give you the step by step instruction to get the gob done with common tools. I believe the sending unit is to the rear and passenger side of the distributor. Good luck and hope this helps. Keep me posted,  be glad to help oh and check the wire harness first, it may have corroded off. 

either your oil pump (inside the engine oil pan) pressure relief relief valve is stuck or the oil press gauge sender is defective. Oil press gauges themselves very seldom are the problem