O Inseong is a South Korean voice actor known for his extensive work in dubbing foreign animated series and films for a Korean audience. He joined the Korean Broadcasting System's voice acting division in 1992 and transitioned to a freelance career three years later.

His voice can be heard across a wide array of animated productions. In the Pokémon franchise, he is the Korean voice of Nyarth, the Japanese name for the character Meowth, in several recent installments. These include the film Mewtwo Strikes Back Evolution and the television series Pokémon Journeys: The Series. He also voiced the character Yellow Bomb in the series Bomberman Bidaman Bakugaiden. In the anime adaptation Demashitaa! Powerpuff Girls Z, he provided the Korean voice for the villainous Mojo Jojo.

Beyond the roles mentioned, O Inseong's career features a broad range of credits, demonstrating his versatility across different genres and formats. His work includes voicing Gon Freecss in the 2011 adaptation of Hunter x Hunter, Beerus in Dragon Ball Z: Battle of Gods, and Wolfgang Grimmer in Monster. He has also lent his voice to various video games, such as Lost Odyssey, and international film dubs, including the Korean version of Ratatouille.