It depends. O-rings usually fail because they've gotten old and cracked, have
taken a set and are distorted, or because there's foreign material on the ring
or the sealing surfaces. It's a little harder to tell if small o-rings have
become distorted (in cross-section) than it is with larger ones that you
usually encounter with UW cameras, but if you do a good inspection of it and
it doesn't look cracked or aged, and seems pretty good in cross-section,
you're probably OK. I've re-used the o-ring on a Mosquito battery door once,
but I wouldn't try to get two battery changes out of it.

Alan

It's probably fine provided that you inspected the o-rings for wear.  I did
this on Mosquito with success.  The mosquito later flooded but not because
of the battery compartment.  The leak was in the face.

I know my LDS sold that battery, oring and plastic backing for a decent
price, so the decision to replace or not wasn't very hard to make.

Chris

One of our divers changed the battery on his Vytec last week and DID NOT
replace the O-Ring. The battery compartment promptly flooded on his next
dive, causing him to lose the display in the middle of the dive.....However
the Vytec has an well isolated battery compartment and the only damage was
to the battery itself ( you can imagine what sea water did to it) . After
replacing both the O-Ring and the battery. it now works as it should. Get an
O-Ring

R