Have a Nissan with oil pressure problems on 1995 Nissan 240SX

The cam pully came off and was replaced and also had oil, filer change and a few hours later the oil filter popped off and lost all the oil. But did not run engine but a few feet before it was turned off. Installed new filter and oil. same thing happens. What is wrong?

Asked by for the 1995 Nissan 240SX
oil pressure is pegged and not being regulated. oil pomp should be checkked by a shop.