Osomatsu, the eldest Matsuno sextuplet, consistently assumes a leadership role among his brothers. During the "Prediction" skit, he joins them in intensely analyzing horse racing forms to pick winners, sparking agitation that escalates into chaotic conflict; the next morning, their positions form the winning race numbers. In "First Time at the Races," Osomatsu accompanies Totoko to the racecourse, intervening when she attempts to mount racehorses by explaining this prohibition. He helps prevent her departure and supports his brothers' efforts to engage her with betting methods.

For the "Check" segment, Osomatsu attempts jockey entry while riding a cheetah, insisting it qualifies as a horse. Challenged, he persists until denied entry. Later, he tries again by having a horse ride him, another unconventional approach violating regulations. At the "Arima Kinen" grand prix, Osomatsu chooses a high-risk trifecta bet, contrasting his brothers' conservative strategies. Though he correctly predicts the winning combination, he fails to purchase a ticket, nullifying his win.

Osomatsu frequently interacts with his brothers during collaborative activities like race predictions and etiquette instructions. His relationship with Totoko involves both guiding her and attempting to accommodate her expectations at the races.