Östlund made his professional debut for
Djurgårdens IF during the
2021–22 season, where he appeared in 11 games. He also appeared in 32 games for Djurgårdens'
J20 team, where he recorded nine goals and 33 assists. Following his selection in the
2022 NHL entry draft, Östlund was signed by the Buffalo Sabres to a three-year, entry-level contract on 16 July 2022. He was returned on loan to continue his development in Sweden with Djurgårdens IF in the
HockeyAllsvenskan. Unable to help Djurgårdens IF gain promotion, after losing the Allsvenskan finals to
Modo Hockey, Östlund left the club and was loaned by the Sabres to continue his development in the top-level
Swedish Hockey League with reigning champions,
Växjö Lakers, for the
2023–24 season on 4 June 2023. After spending most of the
2024–25 season in the
American Hockey League (AHL) with the
Rochester Americans, Östlund was called up to the Sabres on 4 April 2025. Östlund would later make his NHL debut on 5 April against the
Tampa Bay Lightning. While Östlund opened the
2025–26 season with Rochester, he received two call-ups early on as multiple Buffalo forwards picked up injuries. Östlund nabbed his first multi-goal game with two against the
Edmonton Oilers on 17 November 2025. The centre received praise during the season's opening months for his offensive playmaking ability, skating, passing vision and stickhandling. ==International play==