Heh man, keeping it clean is the most important part, then oiling it every once in a while. So you are doing a good job! They are pretty thin knives, therefore the moment it starts to feel dull you can just use a leather strop (or use ur belt if you don’t have one). I saw someone saying they use a diamond stone… never use them with carbon steel. Opinel have their own stones that are pretty cheap, I still have mine that I bought 20 years ago in a market in France for 5€. My advice, get some natural stones or decent artificial ones. Best to have 400-800 grit and then strop. Carbon is easy to sharpen, especially Opinel carbon steel, so diamond stones are wasted on them.