I made an oil pressure alarm from an old Mazda oil pressure switch, an old BMW relay, and an old automobile horn.
Total cost... about $5, back in 1992.
It still works, 15 years on, and would waken the dead.
This is the schematic...
Image of oil pressure switch alarm with relay - Photobucket - Video and Image Hosting
You can wire it in an afternoon, and if anything fails, pull another bit from an old car, and stick it in there.
Remember to fuse the circuit. Use an in-line fuse. They work fine.