Professor Oak is often considered to be the most knowledgeable Pokémon researcher in the Pokémon Universe. He resides in Pallet Town, where he looks after Pokemon that trainers have caught, as well as researching and helping trainers on their journeys. At the beginning of Pokémon Red/Blue/ FireRed/LeafGreen it is Professor Oak who gives you your first Pokémon, as well as a Pokédex. Professor Oak also has a grandson who acts as your rival throughout the course of the game.
In the Pokémon Anime, it is Professor Oak who gives Ash his first Pokémon, a Pikachu, and his very own Pokédex.
Professor Oak will track and reward you for filling up your Pokédex as you catch Pokémon.