Background/Objectives: This study investigated the relationship between spinal canal expansion and clinical outcomes in patients with myelopathy due to ossification of the posterior longitudinal ligament (OPLL) who underwent cervical open-door laminoplasty. Methods: A retrospective study was conducted on 36 OPLL patients who underwent open-door laminoplasty between 2009 and 2021. Preoperative and two-year postoperative radiologic parameters, including bony canal area (BCA) and spinal canal area (SCA), were measured. Clinical outcomes were assessed using the Numerical Rating Scale (NRS) for neck pain and radicular pain, the Neck Disability Index (NDI), and Japanese Orthopaedic Association (JOA) scores. Results: The mean expansion of BCA was 112.1 mm2 (47%) and SCA was 100.5 mm2 (64%). All clinical outcomes improved after surgery, although not statistically significant. JOA scores improved significantly in the severe group, while NDI and NRS-neck scores improved in the mild to moderate group. Significant correlations were found between improvements in NRS-neck and expansions of BCA (r = 0.533, p = 0.001) and SCA (r = 0.537, p = 0.001). NDI improvement was also associated with BCA expansion. No significant correlations were found between canal expansion and NRS-R, NRS-L, or JOA scores. Conclusions: Cervical open-door laminoplasty effectively increased the bony and spinal canal areas in patients with OPLL and myelopathy. In addition to improving myelopathy symptoms, this procedure may also improve neck pain and disability. Further research is needed to assess the long-term outcomes and to better understand these clinical improvements.