Ash's Oshawott is the second Pokémon obtained by Ash during his travels through Unova. Although he is one of the first starter Pokemon to travel with.
Oshawott was one of the Unova Region's starter Pokemon who was under the care of Professor Juniper. When the new trainer, Trip, and Ash who had arrived from Kanto before Trip got here in Nuvema Town witnessed the three Pokemon being shown for the first time, Oshawott made such a good impression that Ash even called it "cute", causing Oshawott to become embarrassed.
Trip chose Snivy which caused Oshawott to become as still as a statue and fall over in shock. Oshawott, however developed an interest in Ash and left Juniper's lab, following the young Trainer from a distance. Oshawott later proved his worth by saving Ash's Pikachu and Iris's Axew from Team Rocket's clutches before fleeing the scene.
Oshawott finally showed himself to Ash and begged for Ash to take him along. Unfortunately, after attempting to capture Oshawott in a Poke Ball and failing, Ash realized that the Oshawott in question was the same one he had seen back at Professor Juniper's lab. Ash contacted Juniper who had become worried over Oshawott's mysterious disappearance and after getting her approval, obtained Oshawott's Poke Ball which gave Oshawott permission to fulfill his dream of travelling with Ash at last but Oshawott had gone missing again. After nearly falling into boiling water while stopping a dam from cracking, Ash saved Oshawott's life by returning the Pokemon to his Poke Ball, resulting in Oshawott becoming Ash's newest addition to his Unova team and also a permanent member of his overall team of Pokemon.
In BW109, Oshawott was used in the Unova League against Cameron's Hydreigon. Despite the notable size difference and ineffective moves, Oshawott was a reasonable opponent for the brutal Pokemon, and hit him right away with Aqua Jet. However, Hydreigon was able to avoid Oshawott's following Razor Shell before striking back with Dragon Pulse. Despite of the hit, Oshawott went on with another Razor Shell, which eventually hit one of Hydreigon's heads. Using the moment it took from Hydreigon to recover from the hit as his advantage, Oshawott used Hydro Pump, which scored a direct hit on the Brutal Pokémon. Hydreigon then fires a Tri Attack, which Oshawott was unable to block, taking more damage. Oshawott fights back with Tackle, but his following third attempt for Razor Shell was countered by Hydreigon's Double Hit, which was then followed by a Dragon Rush, finally defeating Oshawott.
Oshawott is the first of all Ash's starters to not be caught after a Grass-type starter was captured (Tepig was second as always with fire types), Ash captured a grass-starter in each region first, then catching the other starters later on.
Besides Pikachu, Oshawott is the only other starter Ash got from a professor.
Ironically, Oshawott is the Water starter, which is what Ash wanted to start his journey with when he planned to pick Squirtle as his starter.
Oshawott seems to be over-confident, but at the same time, scared of many things.