Gas, temperature, and oil pressure all work off the same voltage regulator in the dash cluster. Since only the oil gauge is malfunctioning, it's probably not the cluster.
With the key on, if you unplug the wire at the sender and momentarily ground the plug to the engine block, the oil pressure gauge should peg to the H side. Only do this for a moment, don't leave it grounded very long!! If the gauge moves ok, you might need a new sender.
If it doesn't move ok, it might be your gauge or wiring that's bad
Don't know why it would fog, unless something's getting hot? Stuff getting hot is never good.