If you have a pair of snap-ring pliers, they make it real easy to take the locking ring off and put it back on. If you do take it off, sometimes it's possible to
gently squeeze the lock ring in a vice (with padded jaws, to avoid scratches) to tighten the fit of it around the pivot ferrule. It may not change much, however; the lock rings have a very strong, springy temper and lots of dimensional 'memory', which means they won't 'move' much either way. But this is likely the safest method to try to fix it. I've done it once or twice with my Opinels, and it's usually just enough to make a small difference.
Opinel's wood handles are also known to shrink or swell a little bit with changes in humidity, or if they get wet. Sometimes that tendency can be used to advantage, by 'moisturizing' the wood a little bit with some oil. That also may not affect the lock ring's fit at the ferrule end, however, because the steel ferrule is also very strong and might not allow much outward expansion either. But it can still swell inwards, toward the pivot. If the wood does swell much, it'll affect the ease with which the blade can be opened or closed. If it swells too much, it may make the blade difficult to open or close comfortably. So, messing with shrinking or swelling of the wood handle might backfire on you, if you're not careful.
Another option might be to contact Opinel directly, and ask if they can send you a new lock ring. There are usually some differences in fit, from one lock ring to another. I figured that out by swapping the lock rings between two of mine to deal with similar issues.
