Osamu Watanabe is the coach of the Shitenhōji tennis club. Watanabe reminds us of a younger (and hipper) version of Yamabuki's Banda Mikiya. Like the latter, the former's interaction with his students is very laid-back, often shown joking around with them. Watanabe is also pretty sly. When the team first learned of Chitose Senri's resignation, Watanabe appeared ambivalent about the issue.
During the match against Seigaku, knowing how much the team needed Chitose and how much Chitose wanted to play Tezuka Kunimitsu, Watanabe "dragged" Chitose back onto the team. This and other incidents show that despite his appearance, Watanabe knows what he is doing. Osamu gave Shiraishi a gold gauntlet to wear until graduation.
Watanabe is shown to be either cheap or poor, telling his team that he'll treat them to somen noodles if they win against Seigaku (as opposed to Ryuuzaki Sumire, who told her team she'll treat them to yakiniku).
- Hobby: horse racing, mahjong
- He gives his team members kokeshis when they win.