What is leftover in the pulp after making oat milk and how can you use it?
Yes, most of the fat, starch and flavour of the oats ends up in the milk, so what you have left is mostly fibre. Dough or batter made from it won't gelatinize much if at all, so will not hold together when cooked like regular oat flour.
Sometimes I add the residue to baked goods (along with wheat flour) in small quantities. It seems to work OK in cakes and soda bread (where a soft texture is desirable), but I need more oil than usual to get a good taste.
I also make quick raw snack balls out of it. I grind 1 part of it with 1-2 parts nuts and/or seeds (usually almonds, walnuts, sunflower seeds) and then I either add dried fruits and/or sugar and sweet flavourings like vanilla, cinnamon, nutmeg, cardamom or dry ginger, OR I add salt, chilli powder and herbs for savoury snacks.
I have also successfully tried mixing it with oats (using about 1 part residue to 3 parts oats) for making oatmeal/porridge or flapjack. I add some desiccated coconut to these dishes to compensate for the lower fat content and blandness of the oat residue.
To be honest, I feel it's not much use as anything but a bulking agent since most of the nutrients and tastiness have been extracted.