Have you considered possibly adding some benefiber or other clear and taste free fiber supplement to the recipe? I use to use benefiber in a lot of the baking I would do for my father to "sneak in some fiber" and I never had to alter the recipe when adding it. I have not tried making the recipe you are asking about (but am going to save it - looks good).
I did find this information if it helps any... "I also use both oats and oat bran in place of all purpose flour. If a recipe calls for three cups of flour, I’m pretty comfortable using one cup of oat bran or oat flour (I just run my oats through my food processor to create my “flour” most of the time) and two cups of white whole wheat flour. I’ve never swapped out all of my all purpose flour for oats/oat bran, but it certainly works well for a partial replacement, especially if you like a bit more of a hearty texture like I do. I love using oats or oat bran in pumpkin breads and cookies – if you like a more “chewy” texture, give it a try." That information was at love from the oven.com in the information on baking substitutes
I have a quick question about subbing oat bran into a recipe. This recipe, in fact. https://recipes.sparkpeople.com/recipe-detail.asp?recipe=1170609
I'd like to add a bit of fibre to this, but I'm not exactly sure how to do it. I've seen it suggested that I use only 1/4c at a 1:1 swap and soak it first. I've also seen it suggested that I add some extra liquid. I'm not a big baker, and I'm new to the whole oat bran thing (right now, I'm putting it in my yogurt because I *loathe* the consistency of yogurt, even Greek yogurt), so I'm hoping someone here can give me some advice!