One that kind of fits is
A Terrible Thing by HerselfBlurb: A couple of years after the final battle in Sunnydale, Buffy interrupts her life of international leisure to travel to Vietnam, for the prospect of seeing someone she never thought she'd see again. Novella-length story complete in two parts. This is part one. Part two is here. Spoilers for all of BtVS and AtS.
Rating: NC-17
Story Notes: I began writing this for a narrative kink ficathon on Livejournal, so long ago that I can barely remember the original challenge. I didn't meet all the criteria, but it was supposed to be set after the end of AtS (check), to involve Oz (check, although he has only a small part), and the narrative "kink" was sunlight. Perversely, I decided to put Buffy in a rainy place. The Vietnam of this tale is a Vietnam of the mind; I make no claim for it to resemble the actual country in any way.
This is an amazing fic and totally worth the read!