I saw some of your earlier posts about carb problems. Could a bunch of fuel drained into the oil and diluted it? I doubt it, but a quick ckeck would be to smell the dipstick.
What kind of filter do you use. Heard about certain brands colapsing and causing a restriction.
I'd check the engine, gauge, and sending unit grounds first. Unplug the sending unit and see if what the gauge does. Short the sending unit wire to ground and see if the gauge pegs one way or other. If it does that should eliminate a defective gauge IMO. In that case you may want to swap the sending unit.
Could the oil pump pick-up have fallen off?
Could the oil pump be defective(stuck bypass valve, cover loose)?
Could be the sign of a spun bearing. I wouldn't be reving it until you find out what the problem is.