Oz calls himself Danny now. He's even quieter than he used to be, and more damaged. The final realisation that he wasn't going to get back together with Willow (and maybe hearing about Sunnydale and assuming that she was dead?) broke him in ways nothing did before that, and ways that it may not be possible to fix.
He's living in a big city somewhere, in a place where he can have a cage installed in his apartment without comment. He doesn't talk even to Devon any more, because all of that is just a reminder of Sunnydale, like the name Oz, and he can't stand it.
Post-Chosen, Willow either deliberately goes looking for him (finds him with the good magic?) or, more likely, happens to stumble across him. She doesn't recognise him at first, but once she realises who he is, she's horrified at what's happened to him. And she decides to try to heal him, but not with magic. She's going to do this the natural way, if she can.
And then, slowly, they develop the kind of post-breakup friendship I would have killed to see on the show, but never really got. Guest appearances from the other survivors, but the core of this story is about Willow and Oz.