anyway this is very bad thing don't drive it untill you figure out what the prob is start by checking the pressure with a mechanical guage right at the sender (located next to the oil filter), remove the sender and put a mechanical guage and see if that reads anythign it is common for the stock sender to go bad. if that dosnt' fix the prob it can be many things that are not too much fun to fix. how many miles? what year? give us something to go on!
If your engine really IS running close to 0 oil pressure, you'll hear the lifters clacking away because they need oil pressure to keep them pumped up. As bj-666 said, verify with a mechanical gauge that you do indeed have an oil pressure problem and not just a bad sending unit.
If your oil pressure really IS low and this dates back to the time when you changed the rear main seal, it's just possible that you might not have torqued down the no.7 main bearing cap to the specified 80lbft and you're losing oil pressure past the no.7 main bearings.
try unpluging your oil pressure sending unit... look at guage and see what it reads than touch it to ground than see what it reads should pin out the guage
if the guage seems to work fine its probably the sender its self