OiKage

OiKage is the slash ship between Oikawa Tooru and Kageyama Tobio from the Haikyu!! fandom.

Canon
Oikawa's a 3rd year and Kageyama's a 1st year, both former teammates on the same middle school team: Kitagawa Daiichi Junior High. Both being the same position: setter, the two have been constant rivals from the start. As Oikawa was Kageyama's Senpai, he was the one who taught him a lot of techniques, like the jump serve, which Kageyama still uses to this day, making Kageyama Oikawa's 'protégé'

They met again in Episode 7 of the first season, now that Kageyama's on Karasuno High School's team. During the 3rd set, Kageyama has a feeling that Aobajohsai isn't using their official setter, and later, Oikawa shows up, after recovering from an injury. Kageyama straight up tells Karasuno that Oikawa's just as bad as him, attitude and skill-wise. Despite this, Aobajohsai High lose and in the same episode, Oikawa sets himself a goal to defeat Karasuno next time, especially Kageyama.

During the match between Karasuno and Aobajohsai, in the Inter-high tournament, Kageyama seems more nervous than any other games he's already played, and this brings him back to his old habits of King, that make him end up on the bench. When he come back to the court he's more trustful towards his teammates and that's one of the changes Oikawa notices in Kageyama. Until the end of the game, the two try to predict each other’s movements, but Seijoh takes the victory. However, they have a rematch during the Spring tournament, Season 2, Episode 20-24, and win. Before they line up, Oikawa goes to Kageyama and tells him not to get cocky, now it's 1-1. The two remain rivals, both seeing each other as someone they can't overcome, but this pushes both players to become better.

Fanon
Although this isn't the most popular ship for both characters, they are shipped by fans. Some stories/fanart show their relationship in middle school, and even though they're in different schools, they have that connection. Others portray them as kings, with their respective colours from their teams.

Dominance portrayal 

Oikawa is portrayed as the dominant one in the relationship. One factor could be that Oikawa's a 3rd year, and also with his personality, he would take the lead. Kageyama on the other hand, is sometimes oblivious and follows Oikawa.

Fandom
FAN FICTION
 * Tooru/Tobio tag on FanFiction.Net
 * Tooru/Tobio tag on FanFiction.Net

Trivia

 * Throughout the series, Oikawa calls Kageyama 'Tobio' or 'Tobio-chan', but Kageyama respects him and calls Oikawa 'Oikawa-san'