I put an OS fs81a in my Osmose. It fits in nicely but I did have to cut a hole in the cowl for the rocker cover to protrude. I also had to position the flight pack as far back as I could under the servo's to get the c of g right. I used a 90 degree header to keep the exhaust system in cowl (there are pics in an earlier post). In flight there is way way way enough power. If I was building again and wanted to use an OS I would probably use the fs72a. I'm sure an FS 91 is possible to fit in but the Osmose really doesn't need that amount of power and I suspect the extra weight and size would compromise it in flight.