Oil pressure warning light
An old-fashioned oil can, usually red
Pull over and switch the engine off · Red means stop
Stop the engine now
Turn the engine off as soon as you can get the car safely out of traffic. Not when you get home, not when you find a gas station. Now, within seconds. If the engine keeps running without oil pressure, it can destroy itself in a matter of minutes, and once that happens the fix is not a repair, it is a replacement engine.
What this light actually means
This is not the same as being low on oil. This light means the car has detected that oil is not circulating under enough pressure to protect the moving parts inside the engine. Oil under pressure is what keeps metal surfaces apart as they move against each other at high speed. Without that pressure, those surfaces grind directly against each other. That is what causes the damage, and why it happens so fast.
Common causes, most likely first
Low oil level is the most common cause, and the simplest one, but it doesn't mean the fix is simple, only the cause is. A car that is dangerously low on oil has usually been losing it somewhere, through a leak or through burning it, and that needs finding.
A failing oil pump is next. The pump is what pushes oil around the engine, and if it wears out or fails, pressure drops even if the oil level is fine.
A bad oil pressure sensor is possible too. Sometimes the light comes on and the engine is actually fine, and the sensor itself is the problem. This is the one case where the light doesn't mean disaster. But you cannot tell the difference from the driver's seat, and treating every case as if it might be real oil starvation is the only safe approach.
Beyond that, clogged oil passages, a bad oil filter, or worn engine bearings can all cause low pressure, especially in an engine that's covered a lot of miles or gone a long time between oil changes.
What it takes to put right
First step is always checking the oil level and condition with the dipstick, right after stopping. If it's low, that tells us something, but the car still needs to come in before it's driven any real distance, because we need to find out why it was low and whether any damage was already done. If the level is fine, the pump, the sensor, and the pressure itself need to be tested on the car to see which one it is. Some of these are quick to sort out. A worn pump or engine damage from running low is a bigger job, and we won't know which one you're dealing with until it's actually looked at.