When mixing wheat flour to make bread, can I add some wheat and oat flakes?
I mix all kinds of things into my bread, so yes, you can do it.
Things to note: if you add things that draw water, you may want to either soak them first (i.e. seeds) or add more water to your dough (i.e. oat flakes, which would loose their structure when soaked).
I don't know how you raise your bread, but in case let the dough raise, then knead again for a second rise:
You may want to put in your additions after the second knead (to make sure your yeast stays undisturbed for the first).