Someone tried to resurrect an old thread from about 20 years ago regarding the age old swelling wood problem. I started to reply not realizing it was so old, but was thankfully stopped by the wise old moderator locking the thread. Here was my reply, nonetheless:
I long ago gave up on trying to seal the wood against moisture. Now I make my Opinels into take-apart knives by filing down the narrow end of the pivot pin. Then I break them down into their five parts, polish everything up, remove the stud on the inside of the Virobloc collar (using either a round file or a Dremel) and sand the inside of the knife where the blade tang rotates. I make them one hand openers that swing almost completely free when the lock is opened. I can open the lock with my thumb and as I swing it I can lock it open or closed all in one motion. This means if the wood does swell, it has a long way to go before impeding the opening and closing to any great degree.
If an Opinel truly gets soaked, I can take it apart and dry it out in the sun (Colorado is as dry as Arrakis) or in the toaster oven.
I truly love tinkering with Opinels!