Roxas didn't always have Oathkeeper & Oblivion. If you remember the fight with Axel in KHII, when he puuls them out, he says "Two?!", as if he's never seen them before.
And also, in this video, at 2:06, 2:10, and 2:29, if you look closely, you'll see that he has the kingdom key. Wh he didn't use THAT in the fight as well, is beyond me. It would also explain why in all the concept art, and pictures of 358/2 Days, he has the Kingdom Key. So, I'm guessing that he finds them sometime in 358/2 Days, most likely at the end. It could also be connected to the XiV member, somehow.
So, I guess what I REALLY doing is hoping you might be able to use them in the online mode.
And also, in this video, at 2:06, 2:10, and 2:29, if you look closely, you'll see that he has the kingdom key. Wh he didn't use THAT in the fight as well, is beyond me. It would also explain why in all the concept art, and pictures of 358/2 Days, he has the Kingdom Key. So, I'm guessing that he finds them sometime in 358/2 Days, most likely at the end. It could also be connected to the XiV member, somehow.
So, I guess what I REALLY doing is hoping you might be able to use them in the online mode.