The more I think about it, the stronger Oro gets. Oro was strong enough to kill the sand kage and the third (granted the third was old). But what's really surprising is this:
Everyone agrees that Akatsuki member are strong. (especially the leader)
So if Akatsuki is that feared, why did Oro just simply leave? It's because he wasn't scared of any of the head members. Why didn't Akatsuki send anyone to kill Oro (hunter nin)? Because the leader probably knew that the members couldn't. Oro definitely knew about their plans because he stated, "if that boy (Naruto) falls into their hands, it will mean trouble for me. Why can't Akatsuki kill him? He's too damn strong.