Background Opioid agonist therapy (OAT) has been shown to reduce opioid use and related harms. However, many young people are not accessing OAT. This study sought to explore how young people navigated OAT over time, including periods of engagement, disengagement, and avoidance. Methods Semi-structured, in-depth qualitative interviews were conducted between January 2018 and August 2020 with 56 young people in Vancouver, Canada who reported illicit, intensive heroin and/or fentanyl use. Following the verbatim transcription of longitudinal interviews, an iterative thematic analysis was used to extrapolate key themes. Results Young people contemplating OAT expressed fears about its addictiveness. Many experienced pressure from providers and family members to initiate buprenorphine-naloxone, despite a desire to explore other treatment options such as methadone. Once young people initiated OAT, staying on it was difficult and complicated by daily witnessed dosing requirements and strict rules around repeated missed doses, especially for those receiving methadone. Most young people envisioned tapering off OAT in the not-too-distant future. Conclusions Findings underscore the importance of working collaboratively with young people to develop treatment plans and timelines, and suggest that OAT engagement and retention among young people could be improved by expanding access to the full range of OAT; updating clinical guidelines to improve access to safer prescription alternatives to the increasingly poisonous, unregulated drug supply; addressing treatment gaps arising from missed doses and take-home dosing; and providing a clear pathway to OAT tapering.