See, here lies your problem: You see Ollie on a cover seemingly just having shot an arrow, and you also see an arrow flying at Black Canary and you assume he shot it at her.
But: The covers, they lie! >_>
Also: I always thought that Ollie is the wide range expert in the team, while Dinah is the close range one, so each of them is better than the other at what they do. Which makes it all the more sad that many writers can't seem to find a way to make this simple separation of duties work in a fight...