Here «ossa» is in the accusative plural, with not a final s in sight. The same desinence occurs with the plural of its longer-vowelled homonym ōs (mouth); «ora». In grammar books, these are generally classified as "3rd declension neuter i-stem" nouns.
Fyi, I don't think os is an i-stem noun. It is simply a normal 3rd declension neuter. An i-stem would be something like animal, animalis, because it becomes animalia (ia) in the plural. It also has "i" for the ablative singular instead of "e".