you'd think it'd be on reverse if it was because of the bull form or human form. oscar is the one with the sheath.
oscar most certainly has the bloodline of poseidon or something. i just don't think poseidon has to be personally involved on his conception. it makes more sense to me that he'd be some lost branch of decendants given the conditions of his upbringing. none of the other mythicals so far were unaware of their kin origins.
there's also the thing with birthing a 7th child. it could be an extra condition for activating the lineage. it'd explain how come there never was a minotaur population. my guess is that you'd have to both be a descendant of poseidon and the 7th sibling.