Not sure if its a bug or not, but you cannot grow Oats. There is neither an option to extract seed from oats nor an option to plant them. its a smaa thing, but it is an annoyance because of the recipes that require it. Oats seem more rare than some of the level 100 specialty items. Only found one dude to sell it.
Looks like there is an issue with the Oat resource that the Essence Trader sells. If you can find the Plant Fibre (Oats) resource, you can use that to cook with and turn it into seeds to grow more for now. (I was able to find some in a chest in an antiquarian forest)