Switch opens with pressure, closes without.
If your test light goes out with oil pressure, your should have it......I'd check for cobbled wiring under dash and or bad pressure switch.
BUT.....I would also get an el-cheap-o mechanical oil pressure gauge and check it for pressure too.......I'd hate to ruin a good Mag18 thinking you were chasing an electric problem when you really had an oil pressure issue!!! Mine all run between 20psi and 60psi depending on oil wt. and operating temp.
Light should be off at around 5 psi, if I recall correctly....