Two Years Prior To The Storyline[]
Oishi and Tezuka initially became friends in their first year in the club. Once on there way home after practice, Oishi told Tezuka that if only first year students were allowed to be a regular player before the summer break. Seigaku would have a chance of getting into the Kantō tournament because of Tezuka's abilitites in tennis. Tezuka then responded that when their time comes, they should definitely lead Seigaku not to the Kantō tournament but to the Nationals. At one point, when Eiji was still a flashy singles player, Oishi played him and defeated him 6-4. From that point on, Eiji decided to play Doubles with Oishi until he could surpass him.
Regionals[]
Prior to the Kantō Tournament, Oishi's wrist is injured during an incident where he saves a pregnant woman from falling. His injury forces Takeshi Momoshiro to play doubles with Eiji Kikumaru. The match that ensues is remarked upon by Hyōtei captain Atobe Keigo to be played not with just Momoshiro and Eiji, but with Oishi, as well - a doubles game with three. In the anime, the next match is against Josei Shonan in which Oishi goes back to the starting line-up and pairs with Kikumaru Eiji in Doubles 1.
Oishi returns for the match against Rikkai Daigaku Fuzoku, and he and Eiji play once more. During this match, the pregnant woman he helped shows up with her husband and newborn baby to watch him. However, they suffer a loss to Niou Masaharu and Yagyuu Hiroshi.
Nationals[]
In the manga, when Tezuka returns from rehabilitation, Oishi sacrifices his wrist (injured during the incident involving the pregnant woman that led to the formation of the Momoshiro-Eiji doubles pair) in a match with Tezuka, thereby offering Tezuka his place on the regulars instead of going through a ranking match.
In the Nationals in the anime, the above event occurs somewhat differently. As the anime had already had Tezuka return, he instead plays a match with Ryōma Echizen, who had left the team at the end of the Regional Select team training camp, and returned in the first episode of the Nationals. The end result is the same, with Oishi giving up his place and re-injuring his wrist.
Although Oishi remains an observer during the match between Seigaku and Higa Chuu, when he observes Tezuka's match against Hyōtei Gakuen, he desires to make a comeback. Oishi re-registers to play with Eiji in Doubles 1, where the two are matched up against Ryō Shishido and Chōtarou Ootori.
The two doubles pairs are practically even and go all the way to a tiebreak, and Oishi and Eiji achieve a new state called Synchro, where they move in perfect unison with one another.
However, Eiji stops and holds Oishi's wrist while in Synchro just before Oishi is about to score the point when he realizes the damage that will be done to Oishi's wrist and forfeits, allowing Shishido and Ootori to win. Once they match is over and they leave Synchro, they forget everything that happened while in Synchro. Oishi along with Kikumaru is rested during the match with Shitenhōji. Oishi returns to action during the Nationals Final against Rikkai Dai in Doubles 1 against Marui Bunta/Jackal Kuwahara pair.
Marui and Jackal easily dominate and quickly go towards defeating the pair however it is revealed that Kikumaru and Oishi can now use Synchro at will and were choosing not to use Synchro until they knew Echizen was prepared to play in the next match otherwise there win would have been pointless. After hearing the news that Echizen could play in Singles 1, Oishi and Kikumaru access Synchro instantly and make a comeback and eventually defeat Marui and Jackal.
Oishi wins the Nationals with Seigaku after Echizen miraculously defeats Yukimura Seiichi in Singles 1. It is revealed in an OVA that Oishi then prepares for his next High School entrance exam. As he tells Kikumaru he will not be entering Seigaku's High School division.
U-17 Camp[]
Oishi along with the whole Seigaku team and several other Middle schoolers was invited to the Japan U-17 Camp. As an All-Japan Junior Representative candidate. All middle schoolers were able to attain a ball that the helicopter dropped which the condition was, those without a ball must go home.
He played a singles tie-break against Eiji Kikumaru and lost, thinking that they were to play as doubles were the loser goes home.
All the Middle Schoolers that lost their tie-break are taken away from the camp by coach however the driver takes a detour and they are met by the Mental Coach who takes them to where the other middle schoolers that lost their tie-break games are. He tells them to climb a mountain by the camp if they want the chance to stay in the camp. Oishi and the 26 other middle schoolers complete this task and end up meeting the High Schoolers who were unable to attain a ball from the helicopter. When they reach the top, they see the eliminated high schoolers training under the Drunken Coach.
Later on, the 25 middle schoolers steal the Coach's sake, angering him to the point he decides they should all go back to the camp and start a revolution. Upon returning to the camp, it is unknown which of the 25 middle schoolers, but some of them defeat the 2nd Court which angers other High Schoolers seeing them return. They quickly handle the High Schoolers with a problem with their return and the Coaches announce they are the new 2nd Court. All of the middle schoolers engage in a pillowfight and then it is announced that the 1st Stringers return to the camp soon.
U-17 Camp Revolution[]
After quick shuffle matches, the Top 20 U-17 players of Japan arrive back at the camp. He played a doubles match with Masaharu Niō against an identical twin high school players from the Top 20 named Mutsu Yūho No.16 and Mutsu Yūma No.15. Oishi and Niou request a doubles match against the identical twins pair Mutsu Yūho and Mutsu Yūma. Niō once again used Illusion and impersonated Kikumaru Eiji thus he was able to use the Synchro with Oishi and took a game from the twins quickly.
However, the twins have more physical strengths and abilities in Synchro. So as the twins become Synchronized, they begin to crush Niou and Oishi. Niou and Oishi, realizing they are about to lose desperately think of a plan. Niou, showing that he had now furthered his Illusion ability so greatly that he could become not only any Middle Schooler but also any High Schooler. Before they became exhausted, Niō impersonated the twins and Synchronized with them. With the three of them Synchronized as triplets, Niou was able to disturb the play by making them play shots and stand in position that would make them vulnerable to Oishi Territory. Oishi and Niou's plan works and the two make a miraculous comeback and defeat the twins making Niou and Oishi the No.15 and No.16 All-Japan Junior Representative.
Pick-Up Contest[]
He and Byodoin are the last two to participate in the Pick-Up Contest. They both fail but they skip out on eating Mitsuya's ricfe balls due the strangers shooting at them. Here, he develops the courage to talk to Byodoin civilly.
U-17 World Cup[]
Group League[]
He and Ochi pair up to play doubles against the first Greek doubles team. He is very nervous. Heracles serves first and hits an ace against Oishi. Oishi seems to be completely outclassed, not even his Moon Volley is working (they just smash it back). Greek takes a 1-0 lead.
Ochi keeps hitting service aces with Mach. The Greek players can't touch Mach and decide to concentrate on attacking Ochi's second serve instead, but Ochi doesn't hit any faults. Ochi finishes his service game with another ace, it's 1-1.
Greece retaliates with a combination play. Heracles jumps in front of the sun, so that Oishi can't see anymore. Heracles pretended to go for a smash, but in reality let the ball pass for Evangelus to finish the point with a delayed attack. Greece takes a 2-1 lead.
It's Oishi's turn to serve. He uses Oishi Territory, so Oishi is up front and Ochi moves to the back. Oishi forces the Greeks to hit lobs which allows Ochi to win points with Mach smashes. The match continues with each side keeping serve until the score is 4-4. Heracles announces he'll start going all out now.