Background <p>Fixed orthodontic appliances pose challenges to maintaining oral hygiene by increasing plaque retention, leading to gingival inflammation. Oral Health Literacy (OHL) is essential for understanding and implementing oral hygiene instructions effectively. Mobile oral health interventions, including smartphone-based programmes, have recently emerged as promising tools to improve oral hygiene behaviours and outcomes. This trial assessed the effectiveness of the World Health Organisation (WHO) mOralHealth programme in improving orthodontic patients’ oral hygiene and OHL.</p> Methods <p>A randomised controlled trial was conducted involving 60 orthodontic patients aged 16–25 in the orthodontic clinic of Faculty of Dentistry, Alexandria University, Egypt. Oral hygiene was measured using the Silness-Löe Plaque Index (PI). OHL was assessed using the validated Arabic version of the Oral Health Literacy Adult Questionnaire (OHL-AQ) across four domains. Baseline assessments (T0) for both OHL and PI were conducted before randomly assigning participants to either a control group (<i>n</i> = 30), receiving standard oral hygiene instructions, or an intervention group (<i>n</i> = 30), receiving the same instructions in addition to the WHO mOralHealth programme (46 WhatsApp messages delivered over 12 weeks). OHL and PI were reassessed after one month (T1) and three months (T2) and compared to T0 to monitor changes in both outcomes. Data collected were analysed using SPSS. The association between PI and OHL was assessed at all time points.</p> Results <p>At T0, no significant differences were observed between groups in demographics, oral hygiene behaviours, or OHL. At T2, the intervention group showed significantly lower median PI scores (1.33, IQR = 1.00-1.63) than the control group (1.83, IQR = 1.50-2.00; <i>p</i> = 0.001). OHL scores were also significantly higher in the intervention group, particularly in the reading and numeracy domains (<i>p</i> &lt; 0.01). Plaque scores were negatively correlated with numeracy (<i>p</i> = 0.012) and decision-making (<i>p</i> = 0.045) within the intervention group.</p> Conclusion <p>The mOralHealth programme implemented via WhatsApp significantly improved oral hygiene and OHL among orthodontic patients over three months compared to standard oral hygiene instructions, with potential for integration into orthodontic care as a practical and scalable approach to enhance plaque control, sustain oral hygiene habits, and support long-term behavioural change when reinforced over time.</p> Trial registration <p>ClinicalTrials.gov, TRN: NCT06734325, Registration date:11 December 2024, retrospectively registered.</p>