I don't know that To'omua is actually too likely to find himself in the XV let alone the 23. Sure he's a good player, but Rennie has sounded as if he'll be picking on Super Rugby form and if the Rebels persist with To'omua at 10, I don't know how much form he will find. To'omua is actually one of my favorite players but every interview from Rennie I've seen he's indicated there really won't be 'incumbents'. Especially considering our past performance that's not an unreasonable position to take.
The other player who might find themselves left out in the cold is Beale, he's yet to look like making much noise for the Waratahs. I think once you start actually looking at constructing a team on form things get really interesting, at this stage it'd be largely the Brumbies and Reds (despite the Reds being 0-3), they are the only teams with any real form, even the Rebels last week kinda muddled through getting the win, but they looked awful doing it.
On O'Connor at 13, I actually think it was a good idea, once you assume you are playing Kerevi at 12, and he was probably our best player if not at least our best back, she he'd be first guy down on the team sheet. But that's the type of player O'Connor is (now) you could play him anywhere in the backline (maybe not fullback?) and he'll give you quality.