so here're some more details. the contract is not very specific. it basically says:
"remove old stairs; frame and install new stairs. cost for pine = $2300; cost for oak = $2950."
of course we discussed very clearly choosing the best wood to match our hardwood floor and that i would do the staining/finishing. so it should've been clear that OSB was not acceptable. but this is not explicitly in the contract. my bad.
since the contract does not directly say what portion of the stairs will be pine, i don't know if i have a strong enough claim in court.
we paid half the money up front. my inclination is to cut the contractor loose. i'll keep the 2nd half of the fee and finish the job myself. it's been like pulling teeth just to get them to come out and do the half-a$$ed job they've done so far. i doubt they'll ever do a good job.
p.s. they installed the stairs ~7" shifted from the old set. now there is a headroom problem too!!