Your engine mounted oil reservoir is probably low. Once the level is drawn
down from completely full it can not refill it's self to the required "brim
full" condition. Find the reservoir of the port side of the engine and take
the cap off and fill completely full. When you put the cap back on oil
should overflow a bit. Unless this tank is filled completely with no air
space, the engine will go to low oil level alarm mode.
Another method to fill this tank is just to loosen (don't remove) the cap
and let the engine idle until the pressurized oil from the main tank fills
the reservoir and starts overflowing pushing oil past the threads on the
loosened cap. This is a Mercury quirk.

--
Ron White
Boat building web address is
www.concentric.net/~knotreel