Haikyuu: Who Is Tooru Oikawa's Love Interest?
They are inseparable and often seen together. Iwaizumi always looks out for Oikawa, and Oikawa puts a lot of trust in Iwaizumi.

Tooru Oikawa is one of the characters of the sports anime Haikyuu. “Haikyu!!” is a story of a boy Hinata Shoyo who is determined and focused on becoming an amazing volleyball player despite his short height. He is the protagonist of the series who is very courageous, cute, and a bit clumsy. To become a great volleyball player, he worked very hard and faced all the obstacles that came in his path.
Within a few months after its release, the anime became a fan favourite. This had a lot of reasons. The storyline, plots, moreover the bond between the characters was the major reason behind this love.
Oikawa was one of the toughest players in the anime. He is fierce, loud, and proud of who he is. He doesn't look for other people's compliments in order to feel good. Oikawa is all about himself. He knows he has girls dropped around his fingers which he finds amusing. But one person, who knew him by heart was Hajime Iwaijumi. Their friendship really was something else. Despite the fact that Iwaizumi found Oikawa annoying sometimes, he sure as hell had a soft spot for him.
Background

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 Daiichi, where Iwaizumi saw 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", and then headbutts him to wake him up. They lost to Shiratorizawa in their years in Kitagawa Daiichi. During the award ceremony, where Oikawa receives the Best Setter Award, they're both in tears, as they are frustrated for not winning over Shiratorizawa in a single match. After those three years, they enter 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. Oikawa's the team captain while Iwaizumi's the vice-captain. In addition, Oikawa's the setter and Iwaizumi's the ace/wing spiker, making the pair have the hitter/setter relationship.
Iwaoi- The Fanship
Oikawa is, similar to the canon universe, popular and charming, often teasing Iwaizumi. 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 since 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.
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. Iwaizumi also looks after Oikawa, telling him not to practice too much and yelling at him when he overworks himself. They care for each other and have always shared a love of volleyball. Iwaizumi is the vice-captain of the volleyball team.
They call each other by nicknames - Oikawa calls Iwaizumi "Iwa-chan" (just like he calls Hanamaki "Makki" and Matsukawa "Mattsun") and Iwaizumi calls Oikawa a variety of insults, including "Shittykawa", "Crappykawa" and "Trashykawa". Neither of them seems to mind the nickname, despite Oikawa often whining "That's mean!" after Iwaizumi insults him.
What Happened To Them In Future?
Post-time skip, Oikawa has moved to Argentina, where gay marriage is legal, and became a citizen there. Many fans joke about how he moved there so he can legally get married to Iwaizumi. This has also been incorporated into fanworks such as fanfiction, with many of them depicting the long-distance relationship Oikawa and Iwaizumi would have, as they live in different countries.
Thanks For Reading!!