Two types of systems, pressurized and not pressurized
I have a 1986 200hp Mercury that has the pressurized system. There are two hoses that go to the big tank, pressure in and oil out. Oil from the big tank is forced into the small tank on the engine so it stays full. If it doesn't stay full, a float switch that is part of the cap sets off the alarm. Make sure that the caps on the big tank seal well, they have to hold about 2psi. There are large O rings in the caps. You also need to bleed the air out of the little tank by loosening it's cap a little until oil starts to overflow and then tightening it. You have to do this with the engine running so be careful, at least that is what the service manual says to do. It is a messy job and you'll need to wipe up a little oil when your done. After you have done this once you'll check the big tank every trip and make sure it doesn't get low again. If this is the system you have good luck.
The other type of system has only one hose going from the tank to the motor. There is probably a primer bulb on this line too. With the motor running pump the oil primer bulb until it is firm and the beeping should stop.