As no "sane" person would promote Osiris to higher O fusions just because of PP, it is better to focus on another Hero (Havoc, Twins, both which can be awakened also and can be used outside PP).
If only Lilith decided to awaken Osiris (as in dot arena), more people may consider fuse her further. She is just too squishy, needs some more health and armor.
And those heroes are considered as floaters, not fliers. Weird but it is like that. Don't think they will change that (only Desmond as he does fly).
I don't know if cara would be good or not but Osiris is definitely good for this fight, even if she doesn't survive until the end. When lumos attaches to her it's pretty much a guaranteed success.
We discussed about problems with old heroes in other topic yet: they are not comparable with new ones (especially against OP) and what's even worse that their awakening doesn't make any difference. So i'm not sure about her and it's obvious that people prefer havoc, twins or any other instead of Osiris.