Iwaizumi, often called "Iwa-chan" by Oikawa, is the latter's childhood best friend as well as the vice-captain of his current team. The two have been playing volleyball together since childhood and are extremely close. Iwaizumi is typically the one person who keeps Oikawa in line within Seijou, as well as disciplines him, usually with kicks or volleyballs thrown to the head (although it's notable that Iwaizumi threatens to hurt Oikawa more than he actually does). Despite being frequently annoyed by Oikawa's antics, Iwaizumi is shown to worry and care about him, often telling him to not overwork himself and cheering him up. Whenever Oikawa feels threatened by Kageyama's talent, Iwaizumi would berate him and remind him of his own strength. During the match against Karasuno, Iwaizumi reminded Oikawa to concentrate on the opponents at hand instead of the upcoming match against Shiratorizawa.
The two are nearly always in complete synchronization with each other on the court. It's well known that whenever Oikawa's in a pinch, he will toss to Iwaizumi.
Oikawa and Iwaizumi have been through a lot together since junior high, Iwaizumi being the one that usually has to look after Oikawa. When Kageyama joined in the two's third years of junior high, Oikawa became jealous of his raw talent and started overworking himself in order to beat him. Oikawa's hatred of Kageyama got to the point where he nearly hit Kageyama, but Iwaizumi stepped in at the last minute. He yelled at Oikawa for his behavior and the two started arguing because Oikawa believed that he had to keep getting stronger in order to beat Shiratorizawa and Ushijima, his longtime rival. Iwaizumi then angrily headbutted and scolded him, stating that he was conceited and wouldn't be able to win against Shiratorizawa alone because volleyball is a team sport and "makes the strong even stronger". This finally got through to Oikawa and changed his way of thinking. It was after this notable change in attitude that Oikawa received his "best setter" reward, and both he and Iwaizumi angrily vowed to beat Shiratorizawa in the future.
After Seijou's loss to Karasuno during the Spring High Representative Playoffs, Oikawa comforted Iwaizumi by patting his back after the former began to question his abilities as an ace.
During the Seijou Extra, Iwaizumi nonchalantly stated that he believed Oikawa would probably not be happy because he's so hard to satisfy, but that Oikawa would keep moving on without hesitation. Iwaizumi said Oikawa is a partner he can boast about, as well as an amazing setter, but that if they fight Iwaizumi will defeat him.
Kageyama was Oikawa's junior when they were in junior high but while Kageyama admired Oikawa, the latter hated him. Saying that he "hates geniuses", Oikawa disliked the raw talent Kageyama possessed and often refused to teach him anything when he asked, stating that it would be troublesome to deal with him in the future as rivals and adding an immature remark back at him. Despite this, Oikawa often comments that he's no match when it comes to Kageyama's accuracy in tossing, which annoys Iwaizumi.
During a practice match in junior high, Oikawa lost his patience and made multiple technical mistakes. He was then substituted by Kageyama, who impressively spiked and tossed. This frustrated Oikawa, which drove himself to work harder but all the while became more frustrated. While in this mood when practicing alone, the young Kageyama asked Oikawa to teach him how to jump-serve, to which he reacted violently and almost punched the boy in the face. He later apologized and was seen showing remorse and guilt over trying to hit him.
Kageyama, however, ended up learning his skills from Oikawa by simply watching him. Since junior high, he believed that if he surpassed Oikawa, he would be the best setter in the prefecture. He holds a grudging admiration towards Oikawa and respects his volleyball skills, but does not care so much for his personality. He is currently trying to master Oikawa's jump-serve.
During Karasuno's first official match with Aobajohsai, both Oikawa and Kageyama get riled up by each others' taunts and jabs. Oikawa likes to purposely agitate Kageyama and throw his state of mind off-balance. Aobajohsai ended up winning this match but Oikawa noted that Kageyama could beat him, just not now.
Sometime later, Kageyama approached Oikawa, asking for advice on how to respond to Hinata's desire to spike "of his own free will" instead of shutting his eyes and hitting with full power. Kageyama stated that Hinata doesn't have the skill and it's a ridiculous proposal. Oikawa originally refused to talk to him, but after Kageyama begrudgingly bowed to show his respect, Oikawa relented and replied that Kageyama should give it a shot. He explained that if Kageyama didn't, he would just be reverting back to his junior high days, and that thinking his way is best is cowardly. Oikawa also told him that it's Hinata who takes the initiative in attacks, not Kageyama.
Ushijima has been Oikawa's rival since junior high, the latter never having won a match against him. Oikawa often refers to Ushijima as "Ushiwaka-chan", much to his disdain. Surprisingly, Ushijima recognizes Oikawa's talent, calling him an excellent player and stating that he should have attended Shiratorizawa because he draws out the maximum potential of whatever team he is on. Whenever the two bump into each other, Ushijima would remind Oikawa of this.