Oil PSI light, H20 temp light?

You can also get adjustable switches and set it for whatever pressures you want it to come on at.
And yes, keyed power to the light and the sender grounds it when the pressure is too low.
Basically what happens is the light would come on when you turn the key on and then go off when your oil pressure came up.

Just an FYI, you can do exactly the same thing with buzzers for an audio warning.
One of my cars had all three (gauge, idiot light and a buzzer) for oil pressure, temp, and charging system.
I had a switch that you could turn the buzzers off or on with if you didn't want audio and just gauges with idiot lights.

Not to long ago on here we discussed how to make the gauges and speedometer backlighting all turn red instead of an idiot light.
That was pretty cool also.