Why should Willow hit or put a spell on him? They parted ways on more or less good terms in "New Moon Rising" and she had - again, more or less - moved on by then. From her point of view everything between them is finished so there should be no hard feelings on her side.
I think she will be confused, maybe even feel a little attracted to him, but nothing will come of it. After all, he was her first love (not counting Xander or Giles here, of course) and that you don't forget so easily even if you're gay now. I'm kind of hoping that Kennedy and Oz will meet as well, because THAT could be interesting, seeing as they're so completely different.
But they definitely - hopefully - won't get back together because... that would be cruel. I loved them together while it lasted but that is past.
The question should be "How will Oz deal?" because that's a different story.