I'm making this Banana Bread and it calls for 3/4 cup of oat flour can I replace it with all purpose flour?

I mean 1/2 cup ap flour instead of 3/4 cup oat flour.

If you have whole rolled oats or quick oats you can make oat flour in the blender or food processor. If you prefer not to use oats or don't have any available, you could use ap flour but you will not need as much. I would start with 1/2 cup additional ap flour, only adding more if the batter is really wet.

You can. Oat flour has no gluten (sometimes it has a trace amount) so be very careful not to over mix your batter or the bread will be tough.