These are great. The sugar / cinnamon ratio is very pleasant. I used 2 cups of oat flour so it was not that fluffy, but still really good.
Edit: I made it again, this time I replaced 1/4 of the oat flour with all purpose flour, and I used olive oil in the batter instead of melted butter. The muffins were slightly fluffier but also much dryer, quite crumbly, so we concluded that the only way to make these is with melted butter.