We examined the effectiveness of occupational therapy intervention combined with standard rehabilitation in patients undergoing high tibial osteotomy (HTO). Overall, 115 patients with HTO for knee osteoarthritis were examined. An intervention group (n = 50), who received occupational therapy and standard rehabilitation, was compared to a control group (n = 65), who underwent standard rehabilitation only (n = 65). Compared with the control group, the intervention group had significantly lower Hospital Anxiety and Depression Scale depression and anxiety scores (both p < 0.05), with between-group differences showing large- and medium-effect sizes, respectively (d = 1.06 and 0.61). Results suggest occupational therapy with rehabilitation during hospitalization improves depression and anxiety rates in patients post-HTO.