Professor Oak is the English name of the researcher who gives the player a starter Pokémon in Pokémon Red and Green. He runs a laboratory in Pallet Town and studies Pokémon alongside his grandson, the player’s rival.
Oak offers a choice among Bulbasaur, Charmander and Squirtle, establishing the series’ traditional three-way starter selection. He also provides the Pokédex, asking the player to record information on the region’s Pokémon.
The character is called Professor Orchid in some early Japanese materials? In the Japanese games his name is Professor Ōkido, rendered as Oak in English. Different localizations can therefore make the naming history seem more complicated, but “Professor Oak” is the standard English answer.
Other Pokémon games introduce different regional professors, such as Elm in Johto and Birch in Hoenn. Those characters perform similar research roles but are not the professor from the original Kanto adventure.