Oshawott originally belonged to Professor Juniper, as one of the three starter Pokémon a new Pokémon Trainer could choose. Ash called him cute when he was sent out in In The Shadow of Zekrom!, when Trip had to decide between the three starters. He then became extremely shocked when Trip chose Snivy. Soon after, he started taking interest in Ash, observing him but staying out of sight, and to the point that he left Professor Juniper's lab to follow him.
Eventually, Oshawott expressed interest in traveling with Ash and convinced Ash to take him along on his journey in A Sandile Gusher of Change!. Ash only realized that he was Professor Juniper's Oshawott when he failed to capture him in a new Poké Ball, and had to obtain a worried Professor Juniper's approval to allow Oshawott to travel with him and to request for Oshawott's Poké Ball. She permitted Oshawott to travel with Ash, but Oshawott had disappeared by the time Ash received his Poké Ball. After nearly falling into boiling hot water, Ash saved Oshawott by successfully returning him to his Poké Ball.
Oshawott was then used against Ash's Gym battle with Cilan and went up against Pansage in Triple Leaders, Team Threats!. Oshawott's inexperience, slow speed, and clumsiness put him at a disadvantage during the battle. However, Ash's quick thinking managed to win them the match when Oshawott managed to deflect Pansage's Bullet Seed and ultimately SolarBeam with his scalchop in Dreams by the Yard Full!. During that decisive match for the Trio Badge, he subsequently used his scalchop in a Razor Shell attack to defeat Cilan's Pansage, earning Ash the victory.
Oshawott was revealed to be male when he battled a wild female Snivy that would be caught by Ash in Snivy Plays Hard to Catch!. He became infatuated when hit by Snivy's Attract, and was eventually recalled.
In The Battle According to Lenora!, he battled Lenora and her Watchog after her Lillipup's Roar pulled him onto the battlefield, to his surprise. Despite fighting well and making use of his scalchop as a shield again, he was defeated by a powerful Thunderbolt after Watchog separated him from his scalchop. In Rematch at the Nacrene Gym!, Oshawott trained with Ash in the Battle Club to increase their speed and power. They trained in a wave pool and after Iris's Axew messed with the controls, Oshawott learned Aqua Jet, but had trouble with hitting the target. He battled Watchog again in Ash's rematch with Lenora after being pulled into battle again by Roar. He still showed some difficulty in controlling Aqua Jet, but he managed to hit Watchog on his third try. Unfortunately, he also knocked himself out in the process after taking a lot of damage from Watchog's Thunderbolt and Confuse Ray attacks, leaving Ash's Tepig to battle Lenora's Herdier alone.
In Emolga the Irresistible!, Oshawott developed a crush on a wild Emolga. When she used Attract on Bianca's Minccino, Oshawott voluntarily jumped in the attack's path so he could get hit with it instead. Later on, he decided to go up against a group of Swoobat to protect Emolga. However, he was soon defeated by a powerful Charge Beam. On both occasions, Ash was forced to recall Oshawott to his Poké Ball. Later in Emolga and the New Volt Switch! he valiantly protected Emolga and Axew from some wild Watchog and retrieved them an apple each, however, a wild Simisear knocked him out, despite Emolga hardly caring.
In Oshawott's Lost Scalchop! Oshawott battled against Stephan's Blitzle, where his scalchop ended up getting kicked out of his hands and off into the distance. Horrified, Oshawott chased after it but, alas, he could not find it. After many failed substitutes, Cilan's Dwebble crafted a new scalchop out of a nearby rock but it ended up being too big and too heavy for Oshawott. Ash then trained him, giving Oshawott the strength to carry the new scalchop. Unfortunately, it ended up being destroyed during a rematch with Blitzle, but Oshawott turned out to be much faster because of all the training and was able to defend himself without a scalchop. He soon defeated Blitzle with a lucky-shot Aqua Jet that sent them both crashing into a boulder. After the battle, Axew found the missing scalchop in the forest and returned it to Oshawott.
In Ash and Trip's Third Battle!, Ash sent out Oshawott to battle Trip's Timburr. Ash commanded Oshawott to use Aqua Jet, which went out of control again. When Oshawott tried the move again, Trip took pictures of him. Trip called a time-out and then showed Ash pictures of Oshawott's face, and revealed that Oshawott's eyes were closed whenever he used it. Trip suggested that was why Oshawott had so much trouble controlling Aqua Jet. The battle resumed, and while both Pokémon used their "weapons" to full extent, Oshawott won out over Timburr using Razor Shell. Following Trip's advice, Ash trained Oshawott to open his eyes under water in Facing Fear with Eyes Wide Open!. Oshawott was at a major disadvantage to a group of Tympole that attacked him while his eyes were closed, eventually, Oshawott opened his eyes and scared off the Tympole. Later when battling the Palpitoad that ordered the first attack, Oshawott executed a perfect Aqua Jet, defeating him and enabling Ash to catch it.
In A Call for Brotherly Love!, Oshawott was used in a battle against Chili's Pansear. After Oshawott volunteered to battle with Pansear, Ash decided that it was a good idea, and the battle commenced. Although Oshawott battled well, hitting Pansear with several powerful Razor Shells, Pansear proved to be too strong and defeated Oshawott with its newly learned SolarBeam.
In Battling the King of the Mines!, Oshawott was used first in the battle against Clay, being sent out to battle his Krokorok, having the type advantage. However, Oshawott was prevented from using his Water-type attacks by Krokorok's Sandstorm, getting sand in his eyes and impairing his vision. Ash was able to get Oshawott back on track by spinning around and using Water Gun, which stopped the Sandstorm and washed out Oshawott's eyes when it rained down on him. With his usual accuracy regained, Oshawott was able to defeat Krokorok. Next, he went up against Clay's Palpitoad, where his Water Gun was far outmatched by Palpitoad's Hydro Pump. Then after Oshawott was hit by a Sludge Bomb attack, he lost track of his scalchop, distracting him from the battle leading him to take a devastatingly powerful hit from Rock Smash and being defeated.
In All for the Love of Meloetta! he met Meloetta and subsequently fell deeply in love with it. To his excitement he was left to watch over it, while Ash and Pikachu left to get more ice. From nowhere, Emolga, who had brought an Oran Berry for Meloetta, crashed into Oshawott and the two began fighting. They calmed down when Ash and co returned and Meloetta began to sing. subsequently in Piplup, Pansage, and a Meeting of the Times!, he fell for Dawn's Buneary, but soon forgot her when she embraced Pikachu and Meloetta revealed herself. When Dawn's Piplup fell for Meloetta just as Oshawott had before him, Oshawott attacked him with Aqua Jet, upset to see Piplup and Meloetta getting along, sparking a rivalry. In the next episode, when the gang were surrounded by Onix and Meloetta appeared frightened, both he and Piplup heroically battled the Onix. Eventually, when surrounded by the Onix again, Oshawott learned Hydro Pump to fight them off. In Goodbye, Junior Cup - Hello Adventure!, Oshawott confronted Piplup over Meloetta one more time before Piplup and Dawn left for Johto, their contempt soon turned into sadness and they bid a teary farewell.
In Meloetta and the Undersea Temple!, Oshawott was training with Pansage, which inspired Meloetta. After transforming into her Pirouette Forme, she insisted that she may train with Ash and Oshawott. However, intimidated and reluctant to fight the object of his affections, he refused to battle. In Unova's Survival Crisis!, Oshawott bid a teary and final farewell to Meloetta, at the aid of Pikachu's comfort.
Oshawott's behavior is similar in a way to many of Ash's past Pokémon, notably Aipom, often attention-seeking and sometimes interfering. This was first shown in A Sandile Gusher of Change!, when he wanted to travel with Ash, he jealously knocked Pikachu off Ash's shoulders and pretended that he was as close as Ash was to Pikachu. Then, when Ash was at the Pokémon Battle Club in The Battle Club and Tepig's Choice! battling another Trainer, he came out of his Poké Ball, even though Ash had already clearly chosen Pikachu. However, he promptly changed his mind to battle when he realized that his opponent was Dewott, a Pokémon of his own evolved form. He has done this many other times, which also shows that Oshawott loves to battle, but only against opponents Oshawott thinks he can win against.
He is also shown to be quite gluttonous, occasionally stealing food from friends and strangers. He mostly did this to Pignite beginning in The Battle Club and Tepig's Choice! when Pignite was a Tepig and continued even after Tepig evolved. This type of action even started a rivalry between the two Unova starters in Baffling the Bouffalant! when they started fighting.