Honestly, I don’t think Oikawa and Kageyama ever achieve a straightforward friendship, even in the most hopeful fan takes. Rivalry’s the engine of their dynamic; the professional respect is there, sure, but friendship implies a level of personal closeness they just never get to in canon. Most fics that try to map them onto a 'rivals to friends' arc either age them up post-high school or place them on the same national team—settings where the immediate competitive pressure is diffused. The shift often hinges on Oikawa finally letting go of his resentment about 'natural genius' and Kageyama learning to communicate like, well, a human. It’s less about becoming besties and more about two intensely driven people recognizing they’re chasing the same impossible standard.
What makes it compelling isn’t warmth, but a kind of bleak understanding. They’re mirrors. Oikawa’s bitterness comes from working infinitely hard and feeling inherently lesser; Kageyama’s isolation comes from being 'gifted' but failing at the human part of the sport. When a story has them bridge that gap, it’s usually through shared exhaustion or a mutual opponent. The 'friendship' feels more like a ceasefire pact between two generals—functional, respectful, but always with the ghost of the net between them.
I’ve read a few that pull it off by making their interactions brutally practical. No heart-to-hearts, just Oikawa offering a piece of cynical advice about managing a team, and Kageyama taking it with a nod. That feels true. Anything sweeter tends to ring false.