My experience with using oak trees that are freshly cut is that they put a lot of acid in your water and that's not good. Let them cure several months then use them for cover. Here is THE BEST SITE EVER for this kind of dialog and information: Pond Boss Magazine Home Page!. Go there immediately. Some of the guys posting here show up there on occasion. You will be amazed.
My neighbor has a HUGE magnolia tree in her yard. I have to go cut the low hanging limbs from time to time. Would these make good structure? Those leaves seem to never decay. Hate mowing near that tree.