Background <p>Traditional assessment in undergraduate oral and maxillofacial surgery (OMFS) internships mostly relies on single-skill examinations, focusing only on technical proficiency while ignoring communication ability, clinical reasoning and medical documentation. This study aimed to develop and pilot-validate a comprehensive competency-based assessment system for undergraduate OMFS outpatient internships.</p> Methods <p>This mixed-methods development and pilot validation study identified six core clinical competency domains through literature review and semi-structured interviews with five senior OMFS faculty members. A multi-dimensional assessment framework was established, including theoretical examination, integrated clinical operation evaluation, phased skill assessment aligned with national licensing standards, patient-related indicators and daily performance evaluation. Forty-six students in the development cohort (2024–2025) adopted the new system; 48 students in the reference cohort (2023–2024) received traditional assessment. Each student was independently evaluated by three faculty members from an eleven-person rater panel. Inter-rater reliability was calculated using the intraclass correlation coefficient. Questionnaire feedback and adverse event data were compared between groups.</p> Results <p>Six core competency domains were finalized. Student ratings of the comprehensiveness of the two assessment systems were not significantly different (8.14 vs. 6.52, <i>P</i> = 0.086). Faculty ratings showed a significant difference in comprehensiveness (9.64 vs. 6.28, <i>P</i> = 0.043). The overall inter-rater reliability was good (ICC = 0.78, 95% CI: 0.65–0.87). Based on participant feedback, the weight of theoretical examination and patient satisfaction was reduced, and more objective indicators were added. No significant between-group differences were found in adverse event incidence.</p> Trial registration <p>Not applicable.</p>