Mary James, thanks for sharing your experience. It's nice to get some first-hand feedback about the use and design of an ozan.
Also, to Miles, I'd like to add to my previous response about the ozan. I had mentioned that I am skeptical about the significance of capitalizing on trapping heat by ozan in the instance of there being a bench/mass that is storing heat and releasing that heat slower than the ozan. The rate at which the warm air can escape the ozan (or least least our particular ozan) is quicker than the mass is releasing warm air, so my thought was that the ozan might not have much meaning compared to the warm mass. I do, though, think that when temperatures are damn cold, that it is damn worth installing all that one can in order to retain heat, so an ozan would be a shame to pass-up. But furthermore, I think the ozan has more than just a heat-trapping role. The liner and the ozan effectively calm draft, and draft is the biggest heat-suck in any home. And no matter how warm a mass (as a bench or stones) is kept, if there's a significant draft, then, well, that kinda ruins comfort. During the winter months when the winds are freezing and life-sucking, the liner and ozan ease the need to stoke the fire because they keep the draft out.
So, from my thus-far experience of being in the tipi, this is how I sympathize with the necessary and/or beneficial things that make this tipi a livable home in a cold climate:
-the RMH is there to make heat
-the barrel is for direct, radiant heat (and cooking!);
-the mass is for heat retention and a slow emittance of heat;
-the outside canvas provides shelter from most elements; and allows for gentle air exchange to feed the RMH;
-the liner and ozan is for protection from draft;
-and the vestibule is to ease the draft upon entering the tipi.
I know those are some pretty rudimentary points, but I don't think they've been outlined in this thread yet. If you have some feedback, or a different view please share.