Periodic Oil Pressure Alarm at around 1800rpm, Volvo Penta D1-30

[CarlF]

If the sending unit is more than 10 years old, replace it. Takes 10 minutes. More than a 50% chance that the problem goes away.

[01kiwijohn]

I see you have a twin engine catamaran. Try swapping the sender units over and see if the problem transfers to the other engine.
The engine's still running so I doubt you have a low pressure problem, although, it wouldn't hurt to put a gauge in there just to verify that.