If your Opinel is one of the standard 'orange' beech-handled versions, it'll need sanding to remove the factory varnish, which is what gives it that orange color. Other premium wood handles from Opinel aren't varnished. Depending on how recently you purchased, I think Opinel is now offering un-varnished beech handles on stainless knives (they don't have that orange hue).
This is my walnut No. 08, finished in Watco Danish Oil (the 'natural' finish, i.e. no dye/stain/colorant). So-called 'Danish Oil' is a mix of boiled linseed oil, thinner and varnish. Does a nice job sealing the wood, and is very durable. Very easy to apply in very light & thin coats, using a folded paper towel to make a 'pad' for wiping it on. I used three applications, with the first two applied about 30 minutes apart, and the last coat was applied after letting the first two cure for about 3 days. This is fairly consistent with the instructions on the container; very straightforward. No sanding needed afterwards, or even between coats:
David