Ōoka is a member of Hayato's clique. He is a member of the baseball club.
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.
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.
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.
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.
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.
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.
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.
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.Sagami rooftop incident. This can be seen when he and Yamato laughed at Tobe's jokes about Hachiman.
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.During the Kyoto field trip he played mahjong with his clique in their room and also proposed that the loser has to get chocolates from girls room but was reminded by Hayama their P.E teacher was watching the halls. He also calmed Tobe down when he was tense about confessing to Ebina.
He accompanied his clique members to the spot where Tobe planned to confess to Hina and was greatly shocked when Hachiman confessed instead. After Hina's rejection of Hachiman, Ooka and Yamato console Tobe.
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.Episode 11 of season 2 (adaptation of Volume 10), he inadvertently strained his friendship with Hayato after he asked Hayato whether he is indeed dating Yukino, which caused Hayama to show his cold side rendering him speechless out of fear but brushed it off as a rumor. He defensively tells Hayato that someone saw them together over the winter break without saying who saw them, or told him. Hayato agrees that it was just a rumour and made peace with him, much to his relief.
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.Episode 12 of season 2 (adaptation of Volume 11), When Tobe revealed his desire for valentine chocolate, both Ooka & Yamato agreed. He expressed his envy of Hayato, who must get lots of chocolate from admirers. He also considers it to be a waste after hearing that Hayama was not going to accept any valentine chocolate.