Professor Oak is the leading expert on Pokémon in the Kanto region. His grandson is Green.
Professor Oak marks his first official appearance in the second chapter of the manga, where he catches Red entering his residence without permission. The professor assumes Red is a thief, and in the confusion, causes Red to accidentally release many of the Pokémon in his lab. After joining Red to recapture them, he becomes impressed with Red's ability to bond with Bulbasaur, and subsequently bestows Red with Bulbasaur as well as with a Pokédex. He later appears outside of Viridian Forest, where he watches Green and Red's encounter, and again via a PC, where he gives Red information on the Moon Stone.
The professor is later seen again via PC, where he tells Red more about his research on the three starter Pokémon, and admits that someone actually stole the third one, Squirtle. Red again briefly contacts the professor via PC, where the professor informs Red of the last remaining HMs he needs to continue on his journey.
Off screen, he is taken hostage by Team Rocket and is tied up in the Silph Company's basements. When Green rescues him, he urges Green to rescue the townspeople of Pallet and reveals that Team Rocket was trying to use the townspeople's ability to commune with Pokémon. After the battle is won, Professor Oak is revealed to have escaped the destruction of the Silph Company building.