The oil pressure sensor is normally closed. Oil pressure while the engine is running keeps it open. The dash startup test checks it's closed. If it was open before the engine is running, you will get a SERVICE warning. If it stays closed after the engine is running, you will get a SERVICE warning, and the light should come one. The dash has smarts to know this, and it's state is tested at switch-on.
As Phang says, it's not a dumb light. The dash controls it.