Professor Oak first appears in Clefairy's Debut!!, in which he calls in Green and Red to his lab to get their first Pokémon. Green beats Red to the lab when they were racing. When both Trainers were in the lab, Oak tells them the news (other than the fact that he is bald) that he is requesting them to go out on a journey to fill up the Pokédex. Afterwards, he gives them each a Pokédex and gives Green a choice between three starter Pokémon, a Bulbasaur, Charmander and Squirtle, in which Green chooses Charmander. Before Green left, a Clefairy comes in. Oak did not allow that Clefairy in the lab as he was known for causing mischief. Clefairy calms Oak down by massaging him, making him forget what happened. As it was Red's turn to choose, Red decides to choose the Clefairy as he enjoyed Clefairy's silliness.
In Will You Risk Your Life to Play the Magic Game?!, Professor Oak tells the two Trainers about a legendary Pokémon, Articuno in the Seafoam Islands, where it made its major appearance in the next chapter. He becomes the announcer and judge in The No.1 Pokémon!! of the No. 1 Pokémon contest, in which he rates the performances of the Pokémon that went on stage, much to Clefairy's annoyance. Professor Oak reappears in Pokémon Baseball Tournament!! as the umpire of a baseball match that Red and Green participate. He tells both Trainers that he'll give a prize to the winner of the baseball match. After a long hard fought match, he gives the prize to Red who ended up winning the event. The prize turned out to be Exeggcute which chase Red and his pals away.
Professor Oak makes an appearance in Clefairy Evolves... At Last?! in which he contacts Red that he is on his way. Red laughs at him, as he doesn't believe that Professor Oak is telling the truth. He shows up and takes Red and the others to a museum which contained fossilized Pokémon. Professor Oak meets up with his friend who works there. He also gets to reunite with the Eevee that he used to have, as it was now use for experimental purposes relating with its evolutions. He watches the Eevee battle with Red's Clefairy and witnessed Eevee evolve into three different Pokémon. Red tells him that he doesn't want this but Professor Oak tells him that it was for experimental purposes and the battle must continue.
Professor Oak in the baseball match
Professor Oak becomes the host of a Pokémon curry competition between Red and Green in Whose Curry is the Most Delicious?. During the competition, he tries out one of Red's meals only to get a shocking surprise which contained a Voltorb.
In Let's Enjoy the New Year!!, he hosts several games that Red, Green, Blue and Giovanni partake. After the games, he is seen relaxing in the hot springs with the Trainers and their Pokémon, save for Clefairy as he got blasted off to the middle of nowhere. Professor Oak goes into outer space sometime before Space Havoc!!. He returns from space and has Red go on a space excursion. After Red arrived back, he gets upset when Clefairy destroyed the shuttle.
Professor Oak makes a cameo in Get that 10 Billion Yen!! (Special Side-Story), in which he tells Red and Green to go into Giovanni's mansion in order to pay off the deal that Giovanni made with him.
In Save the Pokémon Zoo!!, he builds a Pokémon Zoo for his Pokémon to enjoy. When he sees Clefairy, Oak begs Clefairy to make the effort to not use any gag moves to destroy the zoo. However, Clefairy becomes overconfident and gets into an argument with Professor Oak. After Clefairy gets tackled by a Tauros, which sent him crashing into a building, Professor Oak got upset as he knew this was going to happen. However, he gave Clefairy a chance as Clefairy was left off with a warning. Professor Oak watched the shows that Clefairy was hosting in and saw that he did a good job. When the zoo was closing, he felt sad. One night, he notices Red and his Pokémon breaking in. His arrival caused Clefairy to flee in fear, only for him to trip on a hidden pipe which caused the torch to fly out of his hand. When the zoo was set ablaze, Professor Oak along with everyone else was able to get out safely.
In Red VS Green?!, he appears on a television screen telling Red and his Pokémon to arrive in a certain location. In Stop the Missile!! Professor Oak arrives to a control room where he noticed that Giovanni was the one who launched a missile. Professor Oak has the workers do everything they can to stop the missile. Red and his Pokémon volunteer and Oak sends them to a helicopter. When Red stopped the missile, Professor Oak cheered along with the workers. He stopped cheering after finding out that the missile was still able to launch.
Professor Oak in Pokémon Ruby-Sapphire
In Fighting Over Otoshidama!!, Professor Oak was in his house. He leaves his room and finds out that Red, Green, Blue and Yellow are there for the Japanese New Year. He decides to give them money. When Clefairy tried opening drawers, the pouches the money was contained in flew out of the window and Professor Oak had the four boys get them back. After getting the pouches back, he has the four boys go on a rooftop to watch the sun go down and to celebrate the new year.
In The Pokémon Banana League (Parts 1 and 2), Professor Oak, along with the Chairman and Oak's friend, appears as judges of the Pokémon League in the Pokémon Dome. He judged Red's and Green's battle, noticing that both sides's teams got wiped out by each other. Following the match, Red and Green go to Professor Oak's office and he shows them a time machine that will take them back thousands of years. They go in and arrive at the land of the dinosaurs. They land on a Charizard's head and were almost eaten by a Charizard until Red's ancient self throws a rock at it and saves them. Professor Oak goes with Red's ancient self in an old-fashioned car. He along with the group are taken to a palace where the fossilized Pokémon reside. When Red's Pikachu was held captive by Kabutops, Professor Oak decides to take the fossilized Pokémon in his ship and set back for the present.
In Get Lugia!, Professor Oak points to a globe where the locations of the legendary birds are. He has Red, Blue and Green get the orbs representing them in order for Lugia arrive. When Professor Oak goes to the island to check on them, Red, Green and Blue disguise Clefairy as Lugia as Clefairy smashed the orbs. In Good-Bye, Pikachu?!, Red tells Professor Oak about the Dratini he found, Professor Oak and Green explains that Dratini evolves into Dragonair and their orbs are worth 300 million yen each. However, Professor Oak and Green run off after Clefairy tells them that the Dratini didn't evolve yet.
In The End of the Journey!!, Professor Oak throws a party for Red in Pallet Town after defeating Giovanni. However, Giovanni launches second missile and tells all of them. Professor Oak takes Red to the missile site following the launch. He thinks of a plan to stop Giovanni's missile but Clefairy flies on one to stop Giovanni's. Professor Oak watches the missiles explode as they clashed with each other.
Professor Oak in Pocket Monsters DP
After Red's finishes in travels in Kanto and gets reunited with Clefairy, he reappears in Welcome Back, Clefairy!! in a lab at Johto. He tells Red about some new Pokémon he will encounter there after showing him the Marill. Professor Oak goes into another room in the lab where he handed Gold a Cyndaquil and Silver a Totodile as their choice of their first Pokémon. He lays out the Pokégear and Pokédex for the two and gives it to them.
In Marill's Special Training to Surpass Pikachu!!, Professor Oak was eating his lunch and then it disappeared 5 seconds later. He gets upset at Professor Elm but finds out that a Marill was the one who took his lunch. Both professors put this aside and Professor Oak decides to catch the Marill. In Where is Chikorita?!, it is revealed that Green took the only Pokémon that Professor Oak did not give away, Chikorita. However, Chikorita was given back to him in the end of the chapter. In Messing up the Farm!!, Professor Oak takes on Giovanni and his twin brother as they stole five Miltank. After the two bad guys's plans were foiled, he ends up taking the Miltank under his ownership.
In The Hardest Bug Pokémon Wins?!, Clefairy offers a Poké Ball to Professor Oak which he thought it was a Pokémon Red caught, but it turned out to be an onion in the shape of Bulbasaur's bulb. In Showdown at the Radio Tower!!, he has a meeting at the Radio Tower with DJ Mary. However, as the tower was taken over by Giovanni and Team Rocket, they run to the Radio Tower with Professor Oak staying behind. In Celebi, the Green Guardian, he has a lecture about Celebi. Professor Oak has Red and his Pokémon to go out on a hike to search for the legendary Pokémon.
Professor Oak in Pocket Monsters HGSS
In A Formidable Rival!, Professor Oak greets Red and his Pokémon and introduces them to Red. Professor Oak knows that Red is a better Trainer than Red (Isamu Akai). To prove this, Red's (Isamu Akai) Clefairy made coffee for him and spills it on his lab coat. Afterwards, he watches both Reds having a battle with their Clefairy. He intervenes in the middle and admits that Red (Isamu Akai) and the other Red are good Trainers. Professor Oak then gives the other Red a Pokédex.
In A Visitor From Outer Space?!, Professor Oak discovers some DNA in a form of a gag. He discovers part of the DNA to be from Deoxys, and has Red and his Pokémon retrieve the orb. After they got the orb, Professor Oak notices that it was a Voltorb, much to his ire. The Voltorb explodes which causes the building to collapse. In Pokémon Task Force In Action!!, Professor Oak recruits Red and the gang as part of the Pokémon Task Force in order to stop a rampaging Rayquaza. He gives directions on what to do, throughout the telecom on their plane shaped as a Wingull. In Annual Big Cleaning Operation!!, he is revealed to have owned a house in Hoenn. In this one, he left Plusle and Minun in charge while he was gone. After he returned, he noticed that the house vanished.
Professor Oak makes a cameo in the beginning of Find The Ancient Pokémon!! in which he is searching for an Aerodactyl covered in Old Amber. In Spring Cleaning Mayhem!, Professor Oak bought himself a new car. He is visited by Red and his Pokémon in his house. Professor Oak along with Red and his Pokémon unpack in his new house. Professor Oak then goes to his office. When Red and the group checked on him, he wakes up and discovers that the house was still empty as the garbage men took away the moving supplies as they thought it was trash. Professor Oak still had his car only to discover that it was full of water.
In PMDP03, Professor Oak was in a Pokémon Center where he was talking to one of the nurses. Later, the same nurse contacted Professor Oak to tell him what Red discovered underground. Professor Oak and the nurse goes underground and finds out that Clefairy found a sheet of paper with incorrect answers to math problems, much to his irony. In I Want to See a Pro Game!!, Professor Oak eats out and invites Red over. Professor Oak explains to the group about a Pokémon Show going on and takes the group to the Pokémon Show. After the show, Professor Oak asks Clefairy which Pokémon he wanted to pair up with as Clefairy showed an interest in performing. However, he did not choose any of them. In Illusory Pokémon Discovery!, Professor Oak discovers an Egg containing Manaphy. He tries to take care of the Egg but Clefairy did so instead. When the Egg hatched, it inherited Clefairy's personality instead of Professor Oak's. In Big Strategy for the Treasure Gate!!, Professor Oak assigns the job to Red and the gang to collect Time Gears and has Dusknoir to help them out.
Professor Oak goes to his house that he was revealed to have owned in Sinnoh in Big Cleaning Festivities. He hopes that Clefairy doesn't destroy it like the ones he did back in Hoenn. He had the three Sinnoh starter Pokémon to clean up the place and to make sure that the disaster relating with Clefairy's actions doesn't happen. However, Clefairy's idea of having it risen high backfired as the poles supporting it broke. In Turmoil Through the Ages!!, he builds a time machine as part of his attempts to go back in time to see the ancient fossilized Pokémon. He has Red and his Pokémon test it out to see if it works. Red arrives back in the present and Professor Oak noticed that Clefairy went missing, only to age into an old man, much to Professor Oak's surprise. In PMDP36, he has built a house out of grass and used his Pokémon to guard it in order to prevent the house from being messed with.
In PMDP37, Professor Oak does some swimming laps and a Prinplup comes by. He decides to host a swimming race between Clefairy and a Prinplup. After the race, he takes out a swimsuit for Clefairy to put on but it make Clefairy stretch instead. In PMDP55, Professor Oak was racing with Professor Rowan. They bumped into Red's camp set. He witnesses Team Galactic driving away in a van. In PMDP56, Professor Oak is in his house and shows Red and the gang a robot with the word, help, on the chest. He programmed the robot to do many chores that Professor Oak himself cannot normally do. However, the robot backfired after Clefairy altered some controls on it. In PMDP59, Professor Oak made some potions as it was cold and flu season. He tried it on Clefairy but Clefairy made several changes to his work. However, Clefairy ended up getting big.
Professor Oak has returned to Johto as of PMHGSS03. A Team Rocket Grunt tells him a joke which makes him laugh. However, Clefairy's kimono costume ruined it. The grunt still allowed Professor Oak to stay. In PMHGSS08, a bunch of officers were with Professor Oak. Professor Oak reveals his new invention inside a shiny briefcase, which caused the attraction of some Team Rocket Grunts. Red and his Pokémon did everything they can to get the grunts away from them. Professor Oak made a cameo in a flashback in PMHGSS15 in which Cleffa was telling the group a story.
In PMHGSS18, he is the attendant of a contest for which people get to sign up. His job was also to stamp the found locations that was assigned on a sheet of paper. Later, Professor Oak was at his house and some tourists come by. Clefairy manages to help them. He takes some wood from the sign to Professor Oak's house and builds a fireplace out of it. He also takes part of Professor Oak's house to build a stable. Professor Oak appeared in PMHGSS20 in which he along with the other Pokémon professors try to get rid of Red's Clefairy as Clefairy was considered a bad influence towards them.
He reappeared in PMBW19 and PMBW20.