Start with "Use less bran!" - 18.75% of the flour as bran is a LOT of bran.
100% whole wheat (which is hard to get "light" results from) only has 14.5% bran.
Any bran has the effect of cutting the gluten strands and making the loaf rise less. That can be mitigated somewhat by soaking it well, such as putting it in a sponge step before adding the rest of the flour.
If you used oat flour as you say rather than oat bran as your title suggests, it's probably down to needing more water as @Joe's comment suggests. If you're following baker percentage formulas rather than the feel of the dough, you need to stick with the ingredients of the formula you're following to get the expected results; or be ready to adapt if the dough does not feel/behave similarly to the product as done by formula.
I don't use "oat flour" in bread, but I put plenty of oatmeal in, at percentages that high, without negative impacts. My standard wheat flour is 14%, so I've got lots of gluten available. Oats, of course, contribute none, so if you're starting with a lower gluten flour and diluting it with oats, coming up short on the things gluten does for bread is unsurprising. I do add it all at the sponge stage, usually. Since I don't do baker's percentages I can't suggest an amount to change that - I'm strictly a "by feel" baker for going on 4 decades.