Tom, regarding the tires that you have placed, did you drill some holes in the top-most parts?

Someone had previously posted that at first, you want all air to escape, and then over the life of the pond, you want any gases that can build up to be able to escape also.

As far as the oak trees, at their large size, now is the time to do it as it may get to be impossible once the water is in.

You bring up a good point about the "look" of the pond. Some like their ponds to be manicured and very clean looking while others like the more natural/wild look. I think that having some structure break the water surface or shoreline gives visual points of interest as opposed to blights.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."