LOL It is an unexpected combo! Unlike Clark who was developing his identity, he was already Green Arrow on the show (though his alter ego did get his name in the first few episodes). I think the showrunners wanted Batman, but couldn't get him so they settled for the other billionaire playboy who fights crime! Justin Hartley (actor who played him) was initially cast as Arthur Curry for this Aquaman show by the same showrunners but it wasn't picked up so they brought him in as Oliver. Two of the original cast members (Lex & Lana) left after s7 so they needed to fill a gap and well, it worked out for me. I think Oliver being popular on Smallville was the reason "Arrow" as a show came to be. :D