I added some Camino chocolate chips to half the batter, and raisins to the other half. It may not be a good idea to add wet things like fresh fruit. The batter may become too wet and cause the centre of the muffins to have that gloopy texture. Serve your berries on the side instead.
For the oats, you can grind them in a blender, or buy oat flour. 2 cups of oats grinds into 2 cups of oat flour, so if you buy it already ground, just measure 2 cups.
I buy Oak Manor oats because they're grown here in Ontario. I like to support canadian farmers and businesses if I can. Plus, they're organic.
LINE YOUR PAN. This is important. Regular paper liners will stick because these muffins don't have oil or butter. I rarely have good luck removing muffins from the pan with no liner at all, so I just don't do it. The FOK recipe says that you can use silicone muffin pans, but I have had no luck with those either. Maybe they will work for you? I had some paper liners in the cupboard that were made of parchment, but you don't have to buy them, you can make your own. Here's a good tutorial.
These are the oats I use. Oak Manor.