oil alarm sounding

[steplift20]

ok today i went out fishing and my motor oil alarm goes off but it doesnt stay on , it only goes on when i tach out above 3000 rpms and when it does go on it goes off then on then off but the fast i drive the more it stays on
what i did do is when i got back to the marina i took off the oil resevour and topped it off , it took about 8 oz i would guess i also topped off the oil tank but did not take my boat out after that what else could it be and what should i do next
\its a 1988 mercury out board 150 black max with the oil injection still working and i dont really want to remove it