Warning: Spoiler of "Pokemon 4 Ever" follows
As a youth, Professor Oak (or Sammy, as he was called back then) was a Pokémon trainer and he owned a Charmeleon. He met the legendary Pokémon Celebi while it was being pursued by hunters and through its time travelling powers was transported decades into the future for a short while. Professor Oak later trained together with Pokemon trainer Agatha, but their partnership came to an end when he decided he would much rather study Pokémon than train them.
In the present, Professor Oak is an elderly man, roughly 65 years old. His hair is light brown and gray. His usual attire is a white lab coat over a brown pair of trousers and a purple vest. His personality is that of the classic absent-minded professor; a slightly eccentric and not always focused but very intelligent person.
Professor Oak lives and conducts his research from a large laboratory complex situated in Pallet Town, which is based on real life's Yokohama, Kanto region, Japan. His duties include tending to the surplus Pokémon that trainers send him for safekeeping and getting local rookie trainers started by giving them their first Pokémon (a choice between Bulbasaur, Squirtle and Charmander) and a Pokédex.
We know nothing of Professor Oak's parents or children. His grandson, Gary Oak, is a wandering Pokémon trainer and rival of Ash Ketchum, the protagonist of the Pokémon television series. His granddaughter, Daisy Oak, grooms Pokémon in her house in Pallet Town.
In Japanese, Professor Oak's name is Yukinari Ookido-Hakase, a rough translation of which is "Professor Yukinari Orchid, PhD".