<h3>Purpose/Objective(s)</h3> Peer mentorship is the educational, professional, and personal support provided amongst trainees within a similar level of training. While peer mentorship has shown to benefit academic success and professional growth, little data has examined contextual factors, such as curricular change, that may affect the accessibility, content, and quality of these relationships. This study aims to explore peer mentorship among Radiation Oncology (RO) resident physicians following the implementation of a new nationwide, competence-based RO residency curriculum, known as Competency by Design (CBD). <h3>Materials/Methods</h3> Two cohorts of Canadian RO residents in English programs were invited to participate - one being the final academic year before CBD implementation (non-CBD cohort entering residency in July 2018), and the other being the inaugural CBD academic year (CBD cohort entering residency in July 2019). Residents participated via convenience sampling and engaged in semi-structured interviews to elicit their thoughts and perceptions on the impact of curriculum change on peer mentorship. Interviews were conducted until data saturation, meaning that no new ideas or themes were emerging from the data. Interviews were audio-taped, de-identified and transcribed. Iterative data collection was conducted in parallel with thematic analysis methods, using both deductive and inductive analysis, to generate themes to describe findings. <h3>Results</h3> Between April and December 2021, 14 interviews were conducted with 6 non-CBD (32% response rate) and 8 CBD (53% response rate) residents, at which point thematic saturation was achieved. Participants represented 8 out of 10 eligible English RO training programs across Canada. Three major themes were identified: (i) the CBD-cohort described fewer opportunities for peer mentorship in navigating formal evaluation processes, and in discussing uncertainties about the later stages of residency training; (ii) peer mentorship tended to thrive when able to occur as spontaneous in-person interactions; and (iii) there was minimal impact on specialty-specific learning. <h3>Conclusion</h3> Findings from this study identified that inaugural RO residents of a new curriculum experienced uncertainty around new training objectives, and that peer mentorship was most impactful as informal and in-person interactions. Our findings suggest that the unintended consequences of curriculum change on resident peer mentorship may be mitigated from improved orientation and communication about stage-specific training objectives, and to provide increased opportunities for informal activities amongst residents.