The question already seems to be answered but... I use both OSB and plywood for constructing small/medium projects, rarely would you want to use OSB for anything other than interior work. OSB is surely easier to work with than Plywood, but plywood is much stronger and should be used for exterior work like walls roofing. But don't get me wrong their is nothing wrong with using OSB if you are building a small coop with a tight budget, it will still likely last long enough for the short term, but out of all the OSB coops I have seen, most are rotted and cracked (plywood doesn't usually rot but it will sponge out if exposed to weather for to long). Plywood is actually extremely sturdy if used correctly and actually will stiffen up a frame quite a lot, and increase the amount of stress and load a structure can take. I have found quality plywood for around the same price as OSB. Depending on were you purchase you can be paying $8-$30 per 4'x8' piece of plywood board. Over all you get what you pay for. (Extra tip: If you look at the printing on the plywood it usually tells you which side is supposed to be vertical/horizontal for maximum wear bearing performance.)
