The only logical explanation for this situation is that Slade took the opportunity of Moira's candidacy to open the door and make himself known on "The Promise."
We've heard all the evidence, we've seen the tension that could be sliced with an arrow and yet STILL Ollie gets close to everyone but Felicity. Girls will come and go, and Felicity will always be by his side, but she won't wait forever. Stop hiding Oliver!
She was my sister. I couldn't be angry because she was dead. I couldn't grieve because I was so angry, that's what happens when your sister dies while screwing your boyfriend.