Oikawa and Iwaizumi have been best friends since childhood, Oikawa dubbing the latter "Iwa-chan". Due to knowing him so long, Iwaizumi is one of the few who is able to tell when Oikawa is genuinely happy and not faking it for show and is able to tolerate his nonsense for the most part. Though Iwaizumi often insults Oikawa (dubbing him "Trashykawa", "Assykawa", "Shittykawa", etc.) and the two bicker, they work together brilliantly on the court and are completely in sync. Oikawa tends to toss to Iwaizumi if he feels as if he's in a tricky situation. Iwaizumi is often the one to keep Oikawa in check and drag him away from fangirls, much to the latter's dismay. Though Iwaizumi often lashes out at Oikawa for his stupidity, he genuinely cares about him and his health, as shown by him consistently reminding Oikawa to take breaks and not overdo himself. Iwaizumi hates it when Oikawa undermines himself, and angrily reminds him "you're a setter, aren't you!" when Oikawa insists that Kageyama was better at tosses.
Hanamaki and Iwaizumi seem close, though the former is concerned about beating the latter in arm wrestling. He seems to wish to surpass Iwaizumi in strength. He likes to watch (or even join in with) Oikawa teasing Iwaizumi, or the other way around.