Outboard Mercury 115 HP false low oil alarm.

The alarm goes beep, beep, beep and it's really loud!  Besides low oil there are basically three things that can cause this alarm to fail and go off.  The sensor fails, the tank magnet separates from the float or the low oil warning module fails.

The sensor is a bit thinner than a pencil and about two inches long.  It is likely the 1st thing you want to check because it's easy.  With the ignition on (alarm is going off) remove the single screw at the bottom of the tank and pull out the sensor by carefully tugging on the two wires.  It should just slide out.  Once out the alarm may stop.  If it does stop take a strong magnet and put it near the sensor.  If the alarm goes back on you can assume the magnet in the tank has separated from it's float. I've been told this is common and because it's a molded unit you have to buy a new tank:  Part #8628A7

If nothing happens when you pull the sensor and touch it with a magnet it could be a bad sensor or it could be a bad alarm unit:  Part #14857A6

I read many reports of owners who had this problem and just disconnected the sensor or cut wires that later ran out of oil and destroyed their engine which is going to cost you about $8,000.  A new tank (with magnet float) is about $60 bucks and not that tough to install yourself or you could pay $250 to have it done.