I have a few thoughts.
1. If it is a factory electric guage, the sending unit may be bad.
2. ?Where does the needle sit when the engine is not running? ?This may show that the gauge itself may be off.
3. ?How much oil is showing on the dip stick? ?It may have too much oil.
4. ?A trick that some people have used to sell cars is to put a heavy weight oil (50wt, etc.) this will temporarily(sp) cover up low oil pressure.
5. The oil pressure increasing with engine RPM is normal.
...just some thoughts