One of the projects I have finished is sheathing the roof. I ended up using 7/16" OSB and am happy with that decision. It's a lot easier to work with than any thicker material because it's lighter but it is sturdy enough to support the roof. For anyone wondering too, I ended up putting my roof rafters 16" O.C so if you're planning on going 24" O.C I'd recommend something thicker than 7/16". One of my pieces of advice would be to watch the order you decide to sheath the roof in if you have dormers.
In the picture below I have the area where the dormer roof pitch and the living room roof pitch meet circled in red. This intersection of pitches can be tricky if you're not really paying attention(like I wasn't). How I did things was I sheathed the roof section above the front door, followed by the part of the loft(but not the whole thing), then I sheathed the small steep part on the opposite end of the trailer from the door, then I finished sheathing the loft. Because of this I had to deal with the loft roofing intersection in two different ways and one was way easier than the other.
Another thing to keep in mind when sheathing is leaving enough hanging over the edge to cover the fascia board. I am using 1x6's so I left 3/4" overhang from the side of the house so when everything is finished and put together is will be flush and pretty.
After all the sheathing was done we put up some left over Glacier Guard my dad had from building his house years ago. I was pleasantly surprised to see we had enough to cover all of the roof except a 10'x3' section. We used a strip of left over #30 tar paper from the garage to do that bit. I really liked using the Glacier Guard though because it acts as a vapor and moisture barrier and will create a water tight seal around all roofing nails. In short it's ultra water proof and ultra tough. I feel like I have a second roof under my primary roof. The backside is also adhesive so that made applying it much easier for John and I. However we did end up stapling it too to help keep it in place.