O

O is a character of the day who appeared in To Thine Own Pokémon Be True!.

He has a Ping Pong Training Center in Vermilion City. He traveled all the way to Sinnoh to participate in Sandalstraw Town's Pokémon Ping Pong Tournament, which he won. He is well-known for being an expert ping pong player.

After seeing Dawn's Ambipom's talent for ping pong, he invited her to train in his training center. After the tournament, Dawn asked Ambipom if she wanted to continue competing in Pokémon Contests or go practice Ping Pong at O's training center. After much contemplation, Ambipom chose ping pong and left with O.

(Source: Bulbapedia)