I would check the wires, exactly as Don says.
This wire is usually just about the first casualty when the wire insulation starts to deteriorate. My 86 has a bad wiring harness and a new wire spliced in to keep the oil light working. Basically if the wire is missing insulation anywhere, and bare copper touches the engine or frame, it'll turn on the light.

You can find this wire on the passenger side of the engine block, just above the alternator, just behind the water pump, and approximately below the #1 (front) spark plug. It's just a single wire plugged into the side of the block.
It's also possible for it to get trapped when someone changes the oil- I've seen it get caught on the edge of the oil filter.

MAke sure the wire is not touching anything, and if there is bare copper exposed (probably black or dark brown due to oil/dirt/heat), wrap the wire with electrical tape. I only use Scotch 33 tape- it's the only kind good enough for the heat under the hood.

Good luck, and come back here often- this is a great resource for operating and maintaining your car economically.
--
Rob Bareiss, New London CT ::: "Roterande Fläkt Och Drivremmar!"

Assuming that the dipstick shows the right level, more or less, then the problem is likely to be bad wires at the oil switch. This, and the alternator wires, come across the front of the engine in a harness by looping under the front pulley from the left to the right side.

The exposure to heat from the exhaust manifold and to oil from changing the oil filter hastens the degradation of the already crummy wire used in the engine harness.

Most likely is that the insluation has fallen off the wire to the oil sender and the conductors are touching the engine block somewhere near the filter. This will cause the light to glow.

If the wires are OK, then you might REALLY have low or no oil pressure -- best confirmed by a mechanic using a real oil pressure gauge. One cause of this is severe gunking up of the engine (caused by a lifetime of no oil changes). The sludge sinks to the bottom of the oil sump and chokes off the oil pickup screen. (I had this problem with one of my cars when I bought it.)

If the wires look OK, then do NOT run the engine until you can determine whether or not you have enough oil pressure.

BTW, a quick clue is to open the oil cap as the engine idles. If the engine has oil pressure, then you'll see oil slinging off the spinning cam.
--
Don Foster (near Cape Cod, MA)

Most likely cause.
loose connection at the alternator or the oil sender. Check them both, clean them up. Oil sender is on the block behind the alternator.
Karl from OZ
86 245 196K Kms