How to obtain the soft and pillowy texture in bread?
Milk is used as a kind of &quot;tenderizer&quot; in breads. Milk makes a softer crust and otherwise improves the texture of bread, so in this case, I think its your substitution that's giving you a problem. Oat milk may be a reasonable substitute for the liquid required in the recipe, but it just won't have the same effect on the finished product.
If you're not vegan, I would recommend using the regular milk next time. If you are, it may be possible to add a tiny bit more sugar and fat (in addition to the oat milk) to the recipe to compensate for the missing milk sugars and fats, but I'm unsure if that will produce the desired result. You may need to do more research into vegan breads to come up with a substitute for the milk in that case.