Had the same thing on my Cat, full onboard tank and motor tank, but still beeping.
Had a small hole in the oil line causing the sensor to go off.
Quick and easy fix...no problems since.

If you are trying ro prime the in motor tank, leave the cap off the tank when you start the motor and then fill

I had that going on. Noticed the oil was being used out of the upper tank on the engine. Found out the oil from lower tank was not being used so the upper tank was suppling all needed oil. Found out the lid gasket on the lower tank had become hard with age. The way it works is that the lower tank is pressured up and forces oil up to the engine. If the o-ring on the lower tank gets hard and fails to seal no oil goes up. The o-ring looked very good but it was hard and would not seal like a new soft o-ring will. Replaced the old hard looked like new o-ring problem fixed. Stopped pulling oil from the top engine tank problem solved. The top tank had a flat rubber gasket and i replaced it a few days later just because i wanted to. If a one dollar upper tank gasket makes me feel better I'm going to do it too. Just saying but been there done that.

This is a great site. So many tried to help. Awsome

Yea no doubt. The evinrude vro system sucks. However the mercury oiler is a little bit better. And this is an optimax, which can not have the oil injection deleted.

I just had this problem! Check the sensor for continuity, if it has none it's either the sensor or the there's a magnetic ring that sits on the float that activates the sensor. ive been told the sensors hardly fail. if your like me the magnet falls off the float causing your sensor to tell you it's low, and unfortunately you have to replace the on motor resivoir. I hope that makes sense and helps.

Most common is the oil tank float goes bad requiring replacing the complete oil tank. Water in the fuel or the water sensor will cause the 4 beeps also. Had a 115 Optimax doing this found it was #30 Here: http://www.boats.net/parts/search/Merc2/Mariner/90%20DFI%20%283%20CYL.%29%281.5L%29/1B736885%20%26%20Up/Vapor%20Separator/parts.html

I had the same problem. It was the float. I disconnected the alarm and kept the tank full.

I had same beeping with my 2006 optimax. found it to be the fuel filter/water separator. youtubed it and found instruction on draining the fuel line/filter. This was a quick fix but had buy the filter wrench and new filter, that solved problem.

I've had it 2 times on my 02 Opti. Both times it took an new tank like Gary stated.

its an opti cannot premix