Re: xr2/150 mercury.beeping alarm
The motor mounted oil tank must be filled to overflowing or the cap sensor can cause the oil alarm to sound.
Other possible causes are a failing oil pump, the oil pump rotation sensor, or a misfire on the coil that is connected to the oil alarm module.
The alarm module compares the pulses of the pump rotation sensor to pulses from one ignition coil, any difference and the alarm sounds.
Lastly, the alarm module could go bad.
You need to test each component to be sure before ordering any parts as there is NO RETURN on electrical parts.
Check these links for a copy of the service manual, it is THE most important tool you can have.
http://forums.iboats.com/showthread.php?t=309079&p=2072211#post2072211
http://1manual.com/Mercury manuals/Service_Manual_Index.html