Background <p>Medial opening wedge high tibial osteotomy (MOWHTO) is a widely used procedure for treating varus knee osteoarthritis. While achieving an appropriate postoperative coronal alignment is considered important, it is not well established whether optimal alignment actually translates into better patient-reported outcomes.</p> Methods <p>We retrospectively evaluated 203 knees in 173 patients, who underwent MOWHTO. Based on postoperative hip–knee–ankle (HKA) alignment, patients were classified into optimal and non-optimal groups. Clinical outcomes were assessed using the Knee Society Score, Knee Society Functional Score, Oxford Knee Score and WOMAC. Conversion to Total Knee Arthroplasty (TKA) was defined as failure. Surgical survivorship was analysed using Kaplan–Meier method and the log-rank test. Patient Related Outcome Measures (PROM) analysis included only patients who retained their native knee at final follow-up.</p> Results <p>Preoperative and early postoperative HKA angles were similar between groups. However, at a mean follow-up period of approximately 11.5&#xa0;years, patients in the non-optimal group showed significantly greater loss of correction, reflected by a higher ΔHKA (p &lt; 0.001). Among patients who did not undergo TKA, postoperative PROMs were comparable between the two groups. Although the overall rate of conversion to TKA was similar, survivorship analysis showed that failure occurred significantly earlier in the non-optimal alignment group (log-rank χ² = 29.11, p &lt; 0.001).</p> Conclusion <p>While optimal alignment after MOWHTO does not appear to improve patient-reported outcomes, it is associated with better long-term survivorship of the osteotomy. In contrast, non-optimal alignment is linked to earlier failure and earlier conversion to TKA, suggesting that alignment accuracy primarily influences longevity rather than long-term function.</p>