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Background: Motivational Interviewing (MI) is a behavioral approach widely used to improve oral health among adolescents. The integration of face-to-face MI with mobile application support has the potential to overcome limitations related to time and accessibility in conventional care. Objective: This study aimed to evaluate the effect of a combined MI intervention (face-to-face and app-based) on oral hygiene status, knowledge, and tooth-brushing behavior among adolescents. Methods: A randomized controlled trial (RCT) with a parallel-group design was conducted among 100 adolescents aged 13–15 years, who were randomly assigned to an intervention group receiving combined MI (face-to-face and app-based) and a control group receiving conventional face-to-face MI. The intervention was implemented over four months following structured MI stages, with assessments conducted at baseline and three follow-up points. Oral hygiene status was measured using the Oral Hygiene Index Simplified (OHI-S). Data were analyzed using Wilcoxon, Mann–Whitney, and effect sizes were calculated to assess the magnitude of change. Results: The intervention group demonstrated a significantly greater reduction in OHI-S scores compared with the control group. The mean OHI-S score decreased from 3.19 ± 1.16 to 1.37 ± 1.06 (p < 0.001), indicating a large effect size. Conclusion: The combined MI intervention resulted in greater improvements in adolescents’ oral hygiene status compared with conventional MI. These findings highlight the potential of a hybrid MI approach as an effective strategy for adolescent oral health promotion; however, causal interpretations should be made with caution.