Ōoka is a member of Hayato's clique. He is a member of the baseball club.
Appearance
Ooka is shorter than most other characters in the series. He has brown hair and brown eyes. He is usually seen wearing the standard Sobu uniform.
Personality
According to Hayato, he is easy to get along with, always helping other people out and always polite and respectful to his elders and his juniors. Because of that Hayato believes he is a nice person. But Yukino feels that he is an opportunist who worries about his reputation. Furthermore, he can be insensitive sometimes, as he burst out laughing when Tobe belittles Hachiman, which is opposite of Hayato's positive descriptions of him. Hachiman describes him to be a virgin, because he act similarly awkward when the topic of sex comes up.
Abilities
Not much of a focus is given to Ooka but from the perspective of the audience that he is averagely good in sports (at least in baseball). He was also mentioned to have performed in Hina's play. He also participated in the music concert for the Culture Festival along with Hayato, Yumiko, Tobe and Yamato.
Relationships
He is a member of Hayato's clique and is on good terms with Hayato, Kakeru Tobe, Yamato, Yumiko, Yui and Hina. He was shown to chat with them in free time and mentioned to go on outings with them. Hachiman feels that the Hayama clique appears to be close, but he feels their relationship is superficial.
Plot
Volume 1
He was first mentioned as part of Hayato's clique, which Hachiman refers to as the top caste. Ooka often pesters Hayato to do things.
He watched the mixed doubles tennis match between Hayato & Yumiko vs the Service Club. Ooka supported Hayato and Yumiko during the tennis match. He was dumbfounded by Hachiman's last two serves that won the game.
Volume 2
He is one the victims and also a suspect of numerous false chain messages. One of the messages claimed that he played roughly at a practice match just to take out the other school's ace player. Following Hachiman's advice, Hayato didn't pair up with any of his three male clique members. Leaving them (Tobe, Ooka and Yamato) to form their own group. However, they chose to visit the same workplace as Hayato.
Volume 6

He was later shown to perform keyboard in the concert and had a role for the class play during the cultural festival. He along with Tobe and Yamato were bossed around by Sagami to finish the props for Ebina's play.
Volume 6.5
Ooka was shown to participate in the athletic festival. He was on the White team with Hayato, Tobe, and Yamato. He along with Tobe, and Yamato replaced the female broadcasters during the women's athletic event.
Later, he participated in the mens Bo-taoshi event (pole toppling). Following Hayato's command Ooka along with Yamato and Tobe tried to stop Yoshiteru, but was overpowered by Yoshiteru who knocked down their teams pole.
Volume 7
He is frequently shown to chat and laugh with his clique
In the Volume 7 chapter 2 of the light novel, he, Yamato, and Hayato accompanied Tobe to the service club. When Tobe refused to state his request in front of Hachiman as he is "unreliable", he and Yamato giggle in agreement. He, Hayato, and Yamato then leave Tobe at the Service Club alone to give his request.

Tobe nervous before his confession time

Ooka and Yamato schocked at Hachiman's action
Volume 8
Ooka along with Tobe try and keep their clique's spirits up after their relationship became tense. The strain in the clique was caused the day Yumiko, Hina, and Tobe witnessed Hayama and Hachiman's outing with the Kaori and Chika.
Volume 10

Hayama coldly talks to Ooka
At the end of the marathon he and Yamato were shown cheering and clapping for Hayato who won the marathon. He was later shown at Hayato's "victory party" enjoying it with his clique.
Volume 11

The gang wants valentine chocolate
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