I can't address the water, but the Orla to B wouldn't be a bad float, but it's only 7 miles. you could use the bridge at B as a marker and then camp below it and go on to Drynob the second day. I don't think the water below gives you a lot of good camping choices, unless you do the end and take out a Hazelgreen.
No one clears the O fork, so be aware that portage could be required,.