(1) To monitor longitudinal changes in health-related quality of life (HRQOL), pain, knee function, and return to work (RtW) following high tibial osteotomy (HTO) for medial compartment osteoarthritis (OA), and (2) to investigate the influences of psychopathological comorbidities on preoperative impairment and post-operative course. Sixty-four patients were prospectively followed for 24months after HTO to determine HRQOL, pain, functional outcome, and RtW. Psychopathological comorbidities (e.g. depression) were determined preoperatively. Patients with no psychological distress (ND) were compared to patients with psychological distress (PD) in order to investigate the influence of psychopathological comorbidities on outcome. There was a significant increase in HRQOL and decrease in pain from 6month follow-up on. Functional outcomes increased significantly from 12month follow-up on. In general, there was a steady state of outcomes from 6- to 12-month follow-up on. At final follow-up, 90% returned to their previous occupation without limitations. Mental component summary (MCS) and functional outcome showed a positive correlation with RtW, while depression had a negative one. A significantly inferior outcome of group PD versus ND was observed at baseline and early follow-up. RtW was significantly prolonged in group PD (8.9±7.6 vs. 3.9±3months; p<0.001). At final follow-up, both groups presented with comparable outcomes. HTO for medial compartment OA significantly improves HRQOL, pain, and knee function. Time to RtW is high and critically depends on PD. Further, extend of preoperative impairments, an initially inferior course, and inferior MCS outcome was influenced by PD. However, otherwise no significant differences were observed between groups PD and ND at final follow-up. Prospective case series, II.