Need Tree Platform Build Advice
At the time of this post you haven't asked a clear question, so I'll offer some general advice. Your design is reasonable except for a few key things:


Rigidly attaching such a structure to two separate trees is a sure to result in its rapid destruction. Movement of the trees with respect to one another transfers huge amounts of force to the framing. No lumber or fastener will last long. 
Joist hangers serve no purpose in your design. As the common joists rest on the beams, and the rim joists carry little weight, there's no point in using them. Screw through the rim joists into the common joists with coated 3" screws. Toenail through the commons into the beams with same. 
Fastening directly to the tree fails to account for tree growth. Rather, suspend the deck by cables anchored up a ways (from large branches near the trunk, or from brackets bolted to the trunk) in an A configuration. Rather than expensive purpose-built hardware, I'd use some simple steel plates drilled for several lags and a cable eye. 
Keep the primary framing several inches from the trunks, and leave a gap between the decking and the trunk to allow for several years' growth. You can always run a jigsaw around the tree if you need to later. If any of this is unclear, please ask.
OSB isn't an exterior product. Period, end of story. Unless you intend to fully roof and side this thing, don't use it anywhere. 
A solid floor is a bad idea anyway. Think of your floor as a deck, and use decking with drainage gaps. Even if you install the deck boards tight together, the floor will drain and you'll have much less mold and rot. 
To save cost, watch Craigslist and similar for used pressure-treated decking. Ask around for anyone doing a deck rebuild. Often decking that's being discarded is perfectly serviceable once flipped over. I build a round treehouse very much like your plan with some beautiful 5/4 redwood that was headed for the landfill. 


That's all I got for now. I'll update if more comes to mind.