Olasagasti scored his first senior goal on 2 September 2018, netting the C's third goal in a 6–0 home routing of SD Zamudio.[3] He scored seven goals for the side during the season, and was subsequently promoted to the reserves in the Segunda División B in July 2019.
On 1 July 2020, Olasagasti renewed his contract with the Txuri-urdin until 2023,[4] and was a regular starter during the campaign as his side returned to the Segunda División after 59 years.
Olasagasti made his professional debut on 24 September 2021, starting in a 0–2 home loss against SD Huesca.[5][6] Eight days later he scored his first professional goal, netting the opener in a 4–1 away routing of AD Alcorcón.[7]