If in fact your oil pressure is dropping over time, obviously that's not good news. Since the oil pumps are generally pretty robust, reduced pressure will most likely be due to engine wear and increased clearances at bearings etc.
Possibly more likely is a false indication (I'm assuming you are not getting any alarms) I would hope that you'd get a buzzer alarm if any conditions (water temp, oil press, low amps etc) occurred. So if the oil pressure switch is happy, then maybe it's just the gauge itself, or the signal to it. Assuming it's an electrical gauge then you're looking at the transducer as another possibility.
One way to check would be to plumb up a good gauge (say 0 - 100 psig) to the transducer connection and see what the pressure really is. I'd expect anything between 30 - 60 psig depending on temperature. (does your indicator have units on it or just a "normal range"?)
You do need to figure this out.. if indeed you're running low that's not something you should continue to do.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)