You need to isolate the problem area. Usually it is best to reduce the problem area from a large number of possibilities to a smaller number – then do the same again.
Fine your oil light switch. Located on the engine somewhere.
If that is easy to get to then short its connection to ground – this should ensure that the oil light comes on. (If this is hard to get to then select another place where a connection for the oil light is easy to get to.)
If the oil light does not come on then the problem is not the oil light switch but somewhere towards the oil light it self.
Work to a point where the oil light does come on. Then the problem lies between where it came on and where it did not. You can further reduce this by the same method.
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OK?
This method relies on the fault not changing as you test. However as you test you will probably flex the wires and that flex can effect the problem. To check the that fault has not changed it is best to keep the last non lighting 'short' in place, as you flex the wires if the light comes on you know that the fault has changed. This is useful in that you can reflex the wires in a smaller areas until the light goes out – thus reducing the problem area further.
Eventually you may find it is a connection from a wire to a connector (not within the connector .. but from the end of the wire to where it connects to the connector).
Good luck.
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Assumptions;
The oil light works (ie it is ok and the supply to it is ok)
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