Meh - my coop is painted OSB, and going on 5 years old now, and is holding up fine. The OSB was $7 a sheet, treated ply of the same thickness was $30 a sheet. Counting the floor I believe I used 8 sheets. For a small coop for 20ish chickens, I'd never recoup the extra investment.
At some point I'll need to replace the OSB (probably another 5 years), but most likely I'll have replaced the coop with another barn by that point. (The coop also has a flock of sheep and a llama using it as a scratching post, so its not like the OSB is having an easy life)
Good to know, what type of paint did you use? I've been considering using the rest of the "barn and fence" paint I have on mine but I haven't gotten the motivation yet 😊