Opinels are pretty easy to disassemble.
The only thing holding the locking ring in place is the head of the pivot pin, inside the ring. If you hold the knife pivot-up/butt-end-down, looking at the slot where the blade goes, it's on the right-hand end of the pin. Take a pair of narrow pliers (a Leatherman or similar work well) and grab the locking ring under the raised slot on the right side, pull away from the handle, and up towards the pivot, the ring will pop right off.
Next, drive out the pivot pin. I use a narrow nail set, then a sawed-off nail; an old broken drill bit makes a good tool, too. The pivot pin is slightly flared on one end, but is soft enough that it's easy to drive it through the inner collar. It'll hang up badly at the blade tang though; a couple more firm whacks will get it through the blade, and after that, the other side of the inner collar if you want.
A few things I've found interesting:
The blades are stamped out of sheet, so one side of the tang is convex, with a bit of raised metal around the edge. Sanding the tang smooth makes the blade swing more smoothly. If yours is rusty, sanding away the rust will help a lot!
The inner collar is what controls the tension on the blade tang. If your blade is really stiff to open, don't sand the slot bigger, just open up the collar a bit. It's a lot easier to reverse if you go to far! The inner collar is not very springy, unlike the locking collar, you can just wedge it open a bit with the jaws of the pliers you used to remove the locking collar. Reassemble the inner collar and blade, without the pivot pin, to check the fit. If you go too far, just remove the inner ring again and clamp it tighter using a vice or large pliers; check the fit again, repeat as necessary.
Once you've driven out the pivot pin once, it's a lot easer to do subsequently. So feel free to play around with it! For instance, with all the metal removed, it's a lot easier to refinish the handle. Speaking of which...
I've refinished several Opinels; I like boiled linseed oil for a finish, since it repels water well but doens't leave a slick surface like polyurethane. I just sand off the old finish (and tweak the shape to my liking!), remove all the metal parts, and soak the stripped handle in a small bag with the linseed oil. After a few days in the bag, take out the handle, wipe off the excess linseed oil, hand-buff with a clean cloth, and reassemble. Couldn't be easier.