Should be an oil pressure switch mounted on the engine. If you still have the 3 terminal kind, one of the terminals gets a wire to ground; but the earlier single terminal kind grounds through the threads.
On a 73, the hot side of the oil light is kind of strange, as it connects to the brake warning light. The brake warning light in turn gets power from the white circuit (switched but not fused). If the brake warning switch (aka PDWA) is closed, the oil light does not work. Ditto if the brake light is burned out or someone removed the bulb.
When the oil light does work, it is in series with the brake light (they both come on). Maybe it wasn't that way originally, but on my 71 Stag (with the same circuit but different lamps), the brake light is much brighter than the oil light.