The dreaded Mercury beep!
For some reason unbeknownst to most, Mercury utilizes beep codes to tell you when there "might be" or "is" an issue. The problem (as many have found) is that it is very hard to differentiate which beep or code means what.
You think you have a low oil alarm and it ends up being a low water intake alarm.
I would check the oil level to ensure it is where it is supposed to be. I know that on some motors, they like to be right in the middle of the dipstick and higher levels actually throw a pressure code.
When you put it and gear and try to run it, does it go into "limp" mode? Limp mode is "supposed" to be designed so you do not do damage to your motor and you can only get to a certain RPM range. Basically it allows you to get back to the dock.
It could also be a low water intake alarm. This can be caused by a couple of variables. One, it could be a bad impeller. Two, it could be a plugged impeller line or an obstructed line. Three, on some Mercury motors (the Optimax was one) they actually had a pre filter screen prior to the impeller. This pre filter can get plugged with debris and cause the alarms to sound due to improper induction of water.
If it has a flush port, try running your garden hose to the flush port for 10 minutes or so.
The other thing you can do is find someone with a computer, plug it in to the motor and find out exactly what the code is.
I have OCD "Obsessive Crappie Disorder"