After thinking more about your seat, I was trying to think what I could compare the amount of C, that might be in the piece of OSB in your seat, to & the only thing I could think of was the worst smelling hair spray you've ever used.....think about taking that can of hairspray and figure you have a curl end that needs just a touch of spray...you're in the school gymnasium, you spray a little bit on the curl end, walk to the other end of the gym, then walk back to where you sprayed the curl.....the smell would be gone or almost gone because of the transference/exchange of air in the gym.....OK, let's consider your stool seat...it's what, maybe 10" in diameter, figuring the padding & cover, that would make the piece of OSB in it about 8-9 inches in diameter and maybe 3/4" thick.....figuring all of this together, a best guess estimate would put the danger from the formaldehyde at about a 1 in a 1-100 equation, or not very dangerous at all.....is your bedroom big enough for all your stuff, bed, furniture, Kirby's cage...and...a drum set ? ? ?
Something else I just thought of, while writing this post, is if you have any furniture in the house that was put together with twist-lock screws, desk, bookcases or similar, they are probably made of OSB or pressed-wood & was made with an amount of formaldehyde also.....I guess what I'm trying to say is that while the formaldehyde might be detectable in a laboratory, I really don't think what was used to make part of your stool could easily be detected in your room or wherever your drum set is set up.....
