Patient-specific osteosynthesis plates and three-dimensional virtual surgical planning with patient-specific surgical guides have significantly advanced orthognathic surgery, enhancing surgical accuracy. This study compares the outcomes of the mandible-first approach using an intermediate splint and manually bent osteosynthesis plates with the maxilla-first approach utilizing patient-specific osteosynthesis fixation in bimaxillary orthognathic surgery. This multi-centre randomized controlled trial included 88 patients, with 77 completing the study. Patients were randomly assigned to either the 'mandible-first' group (mandible-first with an intermediate splint) or the 'maxilla-first with PSO' group (maxilla-first with patient-specific osteosynthesis). Postoperative evaluation using cone beam computed tomography images showed significantly lower deviations from the preoperative plan in the maxilla-first with PSO group compared to the mandible-first group, for anteroposterior (median 1.0 mm vs 1.8 mm, P = 0.008) and left/right translations (median 0.4 mm vs 0.8 mm, P = 0.003), and yaw rotation (median 0.5° vs 1.0°, P = 0.013). Regarding clinical accuracy (categorized as optimal, good, or suboptimal), 59.5% of the patients in the maxilla-first with PSO group had an optimal or good result compared to 17.5% in the mandible-first group. The study findings suggest the maxilla-first PSO approach offers enhanced accuracy, supporting its adoption in orthognathic surgery for better surgical outcomes.