Can simethicone give a positive result in urine for oxymorphone?

Basically its impossible... Simethicone is an anti-foaming gel that is not absorbed by the body or your GI system into the bloodstream. Its also considered relatively safe & there are no known side-effects. So, No, it certainly shouldn't test positive for anything. Even If it did (hypothetically) the confirmation test would EASILY detect the specific one so no worrys.