Oh man sorry, I just noticed your question so apologies for not replying earlier!
Anyway, I do not know for certain but the older mercruiser 5.7's (properly called a 350 back in the old days) have 2 sending units. The oil pressure sending unit is down by the oil filter
and the warning horn switch is located up by the distributor on the top of the engine
. BTW sometimes this switch is also used to control an electric fuel pump
Anyway, that switch should have continuity to ground with the engine off but have no continuity with the engine running and having oil pressure.
Hope that helps!