alarm on DD 8v92

Short or open at the switch. May be hard to get to if it is intermittent.

Not sure how Ocean Yachts spec'd the oil alarms. Hopefully an Ocean yacht expert will come on.
I'm a Bert guy and know a lil about Hatteras alarms. Not up completely on Oceans.
Again, sounds like a flaky switch.

CF-2 oil has an anti foaming additive specificity for two stroke diesels (Detroits).
Also, over filled oil, the crank can whip up the oil in the sump (pan) and cause air foam in the oil.
Not a real big woo for quick errors but can drive an oil switch crazy.

During your long runs, Does the alarm go away?

Little Low?
Yea, Rite..

A little of both;
Your engines holds about 6 US Gallons of oil each, plus oil for the hoses, cooler and filters.

IMO, if it's not a gallon or so (4 liters), it was not low.
I'm afraid my friend your problem may come back the next time the oil gets hot.

Let me ask another question; Do you have Allison clutches (transmissions)??
If so, has the oil level been changing there?

To help my poor typing skills, lets agree 1 quart equals 1 liter (1.05669 to 1) near 1 to 1.
4 liters to a fat gallon.

You were not low on oil. Your low when it's cold and below the low mark; More than a gallon low.
If one quart really makes a difference, something else is wrong.
Wrong dip stick scribe, Allison front seal leaking, wrong oil in the sump.

You mentioned DD 40w oil. You have not mentioned API CF-2 or CH/J-4 (two stroke or four stroke oil).
Also what clutch you have.

Lots of variables here,,, or just a bad oil pressure switch that worked dock side for you today.

Something is trying to tell you something here. If not completely investigated and cured,,, Well,,, New 8V92s are not cheap.

IMO, 1 quart of oil did not fix anything.

This is not an uncommon issue.

Submit the engines serial number to a real Detroit shop. They will tell you the sumps oil capacity in US quarts.
They may require the angle of the block but on Detroits, usually not.

Your pump out would normally be the old pickup tube from the bottom/aft of the pan. Not the dip tube that enters the block/sump at mid block.
Pump all out in the evening. Do Nothing.
The next morning pump out again, there will be some more oil.
Do not worry about the oil filters or lines, over night they have drained to their correct level.

When all is out, then pump in the spec your real Detroit guy gave you. DO NOT RUN THE ENGINE.
On the dip, this is your new high mark.

Two stroke Detroits you read the dip cold or after at least a hour shut down.
Read the dip warm, any where in-between the hi/low marks is good providing it's not over filled wen cold.

You have still not responded what clutch you have or verified the API rating of your oil.

If your oil is old in terms of hours it gets thinner; this is a common cause for the oil alarm to sound. Note it most likely happened first at idle after the days cruising at higher rpm. Then more and more often. Change the oil and let us know if it still happens. Removing all the oil you can and changing filters does not give an accurate amount of oil to put back in- because the filters hold oil. On my 16-92's I'd pull out 16-17 gallons per side but put in closer to 18. That's my experience running a set of 16-92's for 11 years in a sportfish. Use an oil with a CF-2 rating ONLY.