Is my buff orp a ROO??

[eggHaus]

Hello! I have a 15 week old buff Orpington that recently got introduced to my established flock along with a RIR the same age. I got them both at TSC on the same day. Today, the buff orp has been really mean to the other chickens. Grabbing feather at the back of their necks. Then she got on top of my little Silkie while biting her neck feathers. Is this just dominance behavior and establishing the "pecking order" or is she maybe a he?? Here are some pics of the buff Orpington in question along with the same age RIR.

 

[TopRooster]

It looks like a rooster and that behavior means he might start mating

 

[Chicken Girl1]

Definite cockerel and the behavior is he is trying to mate.