I'm tweaking the recipe I have for bread, as I'm always trying to make it better. It calls for 4c of ap flour, and uses honey rather than white sugar (there is no milk in it). I also use refined coconut oil (no coconut taste). Up to now, it's been quite satisfactory. However, next time I bake, I'm going to use 3c ap flour and 1c oat flour and will also include 1 whole egg instead of the tbl. of apple cider vinegar I usually use. If I add the egg, do I reduce the amount of liquid used and by how much? Oh, I'm not using a bread machine. I'm baking for just me, and usually get a decent loaf and 4 buns from the dough. Thanks much for your help!
Since this is a bread you are accustomed to baking, you know how the dough should feel. You will either have to change the amount of liquid or the amount of some other ingredient(s) - probably the flour. I suggest that you start by adding some of the flour to the oat flour and all of the wet ingredients. Then, continue adding more flour until the dough reaches the proper consistency. Add flour in consecutively smaller amounts, kneading as you go.
I assume your dough has water in it. Some bakers would start by adding the water to the wheat and oat flours, and let it sit for about a half-hour before adding the other ingredients. This allows the flour to hydrate. If this results in the dough you're after, you're golden! If not continue, to knead in flour in small amounts until you reach the proper consistency.
This post has a good description of adding flour to wet ingredients.