IwaOi

IwaiOi is the Slash ship between Iwazumi Hajime and Oikawa Tooru from the anime Haikyuu!!

Canon
Iwaizumi and Oikawa have known each other since they were children. They started to play volleyball together, so naturally Iwaizumi witnessed Oikawa's growth as they both entered Kitagawa Daichi, where Iwaizumi will witness Oikawa over-exert himself for the sake of surpassing Kageyama. During that time, Iwaizumi scolds Oikawa for overworking himself and for forgetting that "the better six are stronger", then head butts him to wake him up. They lost to Shiratorizawa in their years in Kitagawa Daichi. During the award ceremony where Oikawa receives the Best Setter Award, they both are in tears as they are frustrated for not winning over Shiratorizawa in a single match.

After those three years, they enter into Aoba Johsai High, where they continue to play volleyball together. In the timeline portrayed in Haikyuu!! when they have their debut, they are in their third year, with Oikawa as the team captain and Iwaizumi as the ace.

In the extra chapter in Volume 17 of Haikyuu!!, they thank each other for "these three years" after they go to their usual ramen shop with their teammates.

Dynamics
They are inseparable and often seen together. Iwaizumi always looks out for Oikawa, and Oikawa puts a lot of trust in Iwaizumi, causing them to be a formidable team in volleyball as they have near-flawless dynamics and trust in each other.

Fanon
In most IwaOi fics, Oikawa is, similar to the canon universe, popular and charming. Iwaizumi is most commonly portrayed as Oikawa's childhood friend. They are inseparable as friends, and also as a couple. Oikawa often overworks himself, and Iwaizumi looks after him by paying close attention to his health and furtively leaving signs that he cares. Iwaizumi is often portrayed like he does not care about Oikawa on the outside, but he actually cares about him deeply, and will do anything and everything for him.

A common dynamic between them is Oikawa misunderstanding Iwaizumi, thinking that he is just here because he has been with him from childhood, and not because he has any romantic attachments to him. But of course, it is the contrary—Iwaizumi cares for Oikawa very, very deeply. He will do anything to protect him.

Another common dynamic between them is Iwaizumi pining for Oikawa. In the canon universe, Oikawa is very popular with girls—it is distinctively similar in the fanon universe. As Oikawa dates more and more girls, Iwaizumi finds himself disappointed and hurt when he sees Oikawa with his girlfriend. Their relationships don't last long in most cases, and Oikawa dumps the girl or vice versa. Then, it is revealed that Oikawa is trying to distract himself from Iwaizumi, but eventually fails to.

In the Final Haikyuu Quest universe, Oikawa is the Grand Demon King, while Iwaizumi is a knight and an ally of Hinata. They meet in the final boss battle, where Iwaizumi reminisces their memories from their childhood—this does not end well in most cases; very often do they find themselves grieving the other.

Their main troupes are misunderstandings, mutual pining and slow burn. Angst is always welcomed in their fics, even though there is also a remarkable amount of fluff. They are also often paired with another couple, MatsuHana (Matsukawa Issei and Hanamaki Takahiro) in fanfiction.

Trivia

 * They play on a team with Yahaba Shigeru, Kyoutani Kentarou, Matsukawa Issei, Hanamaki Takahiro, Kunimi Akira, Kindaichi Yuutarou and Shinji Watari.
 * Oikawa busted his knee from overworking himself.
 * They are one of the most popular pairings in the Haikyuu!! fandom.
 * Together with Matsukawa Issei and Hanamaki Takahiro, they make up the Aoba Johsai third years in the volleyball team, most commonly known as the 'Seijou Third Years'.
 * 'The fight isn't over', an extra chapter in Haikyuu!! volume 17 is a Very Important Chapter™ to IwaOi fans.
 * In an extra four-panel comic strip in Volume 17, they are shown to be quite competitive with each other as when they race each other, neither of them were willing to let the other take the lead.
 * In a lot of IwaOi fanfiction, the Seijou third years frequent a ramen shop where they eat together. (Oikawa pays in most cases.)
 * They have the dynamics of the King and his Lionheart.
 * Oikawa often makes fun of Iwaizumi for his height.
 * Oikawa likes aliens (as shown in panels where he is a child, he often wears shirts with aliens on them)
 * Iwaizumi is often portrayed to be a big fan of Godzilla in fanfiction, as he has a Godzilla phone case (as seen in the manga). In Let's Haikyuu!!, he is also portrayed to be so.
 * This ship is most commonly known as IwaOi.
 * Iwaizumi has a tendency to hit Oikawa.
 * Oikawa often calls Iwaizumi by his nickname, 'Iwa-chan'.
 * They are often portrayed as the 'dream team' of the Seijou third years, with Hanamaki and Matsukawa as the 'meme team'.
 * [SPOILER ALERT] In their last match against Karasuno before graduation, Oikawa's last toss was to Iwaizumi.
 * [SPOILER ALERT] They go to different universities after graduating from Aoba Johsai.
 * The person who wrote this article hated the original one so much they made an account on Wikia just to rewrite this. I also stayed up for an hour to write this. Whoever wrote the original one, I'm so sorry but I'll need to ramble about my favourite ship