This happens when there is a blockage in the oil pan on a SOHC V6, there is an issue with the oil pressure sender or you actually don't have any oil pressure. The first two possibilities are not uncommon. I've had this happen on 2 Explorer's (a SOHC and a V8). The reason it happens is that the oil pressure sender is just an ON/OFF switch. If the sender malfunctions (or if you have intermittent oil pressure due to a blockage) the gauge's needle bangs up/down so furiously that it pops over the peg.
You could go through the trouble of removing the lens from your instrument cluster and lifting the needle over the peg, or you can drill a 1/8" hole in the lens just above where the needle is sitting, insert a small pic and lift the needle over the peg.
I prefer to just drill the hole as it will likely happen again at some point. The plastic top of a Bic pen can be used to plug the hole in the lens afterwards if you like.
If there is nothing wrong with your oil pressure you should replace the pressure sender. It's located behind the power steering pump on the side of the engine block. As you have a SOHC V6 you should drop the lower oil pan and look for broken pieces of tan plastic timing chain guides which are blocking the oil pickup.
