Oliver and Sara turning to each other has always made sense to me. It's not what I would have chosen, and in fact I wished and predicted it had been left at hooking up once, but I understood it. Oliver just wasn't ready for Felicity and must have thought they were even more unthinkable than she did, and he and Sara got each other and didn't fear that they might sully the other person. My one complaint is that their team ups took over the show for a while.