Background <p>Oncology nurse navigators play a critical role in addressing patients' physical, emotional, social, and spiritual needs. However, gaps in existing training programmes often limit the effective integration of holistic care principles and therapeutic relationship skills into clinical practice. Online education offers potential flexibility and scalability, yet evidence regarding its effectiveness for developing these competencies remains limited.</p> Methods <p>This study employed a quasi-experimental design comprising two sequential phases of online training aimed at bridging theoretical foundations and practical application in oncology nursing navigation. Healthcare professionals across Taiwan participated in Phase One, which consisted of online dialogic seminars focusing on foundational concepts of holistic care. Phase Two involved situation-based simulations and structured reflective discussions to strengthen applied skills. Holistic care competence and therapeutic relationship skills were assessed at baseline (T1), mid-training (T2), and post-training (T3) using validated self-report instruments. Data were analysed using paired t-tests and repeated measures ANOVA.</p> Results <p>A total of 337 healthcare professionals completed Phase One, of whom 34 oncology nursing professionals met eligibility criteria and participated in Phase Two. Statistically significant improvements were observed in holistic care competence, with paired t-tests indicating significant gains from T1 to T2 across all dimensions (<i>t</i> = − 9.65 to − 13.00, all <i>p</i> &lt; .001). In Phase Two (n = 34), repeated-measures ANOVA confirmed significant improvement in holistic care competence across T1, T2, and T3 (<i>F</i>(1.50, 49.47) = 14.83, <i>p</i> &lt; .001, <i>η</i>^<i>2</i><i>p</i> = .310, Greenhouse–Geisser corrected). Therapeutic relationship skills also improved significantly (<i>F</i>(1.35, 44.56) = 12.18, <i>p</i> &lt; .001, <i>η</i>^<i>2</i><i>p</i> = .270), with the majority of improvement occurring from T1 to T2 (<i>d</i> = 0.59, <i>p</i> = .002). Effect sizes from T1 to T3 ranged from medium to large across holistic care dimensions (Cohen's <i>d</i> = 0.81–1.07).</p> Conclusions <p>The findings suggest that an interactive online training programme incorporating scenario-based learning and reflective discussion was associated with enhanced holistic care competence and therapeutic relationship skills among oncology nursing professionals. Baseline therapeutic relationship skills predicted learning gains across all holistic care dimensions, indicating that relational competence may function as a foundational infrastructure for holistic care learning; accordingly, positioning relational skills training as an early component of oncology nursing curricula may optimise learning outcomes across all dimensions of holistic care. While the results are context-specific, this approach demonstrates potential as a feasible and scalable model for advancing professional education in oncology and palliative care settings.</p>