Greeting from down under!
I have owned my Merc 2.5ltr EFI 150hp Limited Edition (Retro) 25" outboard, since new in the carton.
A few months ago the Oil alarm was sounding. I discovered the cap on the reserve oil bottle under the cowl had popped up about 2-3mm and was leaking oil. I retightened by hand, started the motor and within five minutes the alarm sounded again. I removed the cowl to find the cap had popped up again. I retightened it by hand for the second time, to discover the thread of the reserve bottle was stripped.

I located a used bottle and fitted it... problem solved! Yeah.
After about 6 outings in my boat and say 6 engine ours, the alarm sounded again. I immediately checked the oil level of the primary oil bottle located in the boat, the bottle was at least half full. However, I did notice two pronounced white stress marks on the front face of the primary oil bottle, plus it was very puffed up.
I removed the cowl and checked the cap of the replacement reserve oil bottle. The thread of that bottle was stripped, exactly the same as my original bottle.
Is there a possibility of something causing over pressurization of the Auto Oil System?