I happen to like Osiris quite a bit, but she's just not QUITE there yet (even with the newest buffs) to carry Dark. If she ever gets an AW that slightly strengthen her skills (and/or shorten CDs), she could very well become a core member of dark going forward similar to how Michael is to Light. However, in the current, Dark is still missing some pieces to make such a team work well, so she's not really the best fit atm, IMO. Still, that is more a problem of Dark as a whole rather than Osiris specifically.