Shuichiro Oishi (大石 秀一郎, Ōishi Shūichirō?) is the vice-captain of the club. Along with Kikumaru Eiji, he is part of Seigaku's national level doubles team, known as the 'Golden Pair'. Oishi is known as the 'Mother Hen of Seigaku', because he constantly worries and frets for the team's well-being. Oishi is calm and rational in a match, allowing him to form strategies and guide the match. He uses the "Moon Volley", a topspin lob that travels very high over the court and lands just in bounds. He can also use the "Drop Volley", a feint hit with the power of a drop shot, making the trajectory much closer to the net. He is voiced by Takayuki Kondou in Japanese media, voiced by Sam Riegel in English media, and portrayed by Hiroki Suzuki in the live action film.

They often use the "Australian Formation", which involves standing directly on the center line and confusing the opponents as to which way each player will move when the ball is placed into the opposing court. The "I-Formation" is a variation where Eiji stays below the net and Oishi serves the ball down the center. They also develop "Oishi Territory", which has Oishi focus on the net, while Eiji stays in the back. It takes advantage of Oishi's ability to control the flow of the game and Eiji's Seal Step. They later develop the "Synchronization" ability, which lets them play and move as if they are one person.

Oishi is also a "Arbiter of the Grill". On the episode "Prince of Yakinku, Oishi turns into a totally different person. He thinks that grilling is the point of Yakinku. Also, he yells at the team, giving them instructions on how long they should grill it.Later on he sabotoged by Eishirou Kite, the captain of Higa Chuu by adding hot sauce in Oishi's sauce).

Oishi also plays the mother in the tenipuri episodes. Inui is her husband there, and Echizen Ryoma, Kaoru Kaidoh and Takeshi Momoshiro(Momoko) are her children.