Professor Samuel Oak, a celebrated Pokémon researcher and retired Trainer, guides Ash’s journey in the Sinnoh region during the Diamond & Pearl series. Known in his youth as "Flying Finger Sammy" for his video game prowess, he abandoned training after discovering an Omanyte fossil, pivoting to academia. As a former Celadon University lecturer, he mentored figures like Professor Elm and Delia Ketchum while pioneering innovations such as the Pokédex. Throughout Ash’s Sinnoh adventures, Oak provides tactical guidance, urging exploration of sites like the Old Chateau and collaboration with Top Coordinator Johanna. He visits Sinnoh to present lectures, including a Twinleaf Festival talk on Evolution stones’ symbiotic relationship with Pokémon. Mid-journey, he aids a Lombre by gifting a Water Stone, triggering its evolution into Ludicolo, thwarts a disguised James impersonating him, and diagnoses a Tyranitar’s mysterious condition. Oak’s Pallet Town lab houses Ash’s reserve Pokémon, while his own team reflects his fieldwork—Rotom, captured during M’toro Island studies, and Dragonite, deployed to neutralize threats like a rogue Mewtwo. His evolutionary expertise shines in analyses of Accelgor and Escavalier interactions during joint battles. Beyond academia, Oak judges Contest Battles, leads seminars, and aids fossil restoration initiatives, reinforcing his dual dedication to scholarly advancement and hands-on mentorship for Trainers across Sinnoh.