Few studies have identified the effects of arthroscopic surgery on the clinical outcomes when open wedge high tibial osteotomy (OWHTO) and arthroscopic surgery were performed together. The purpose of this study was to evaluate the clinical efficacy of arthroscopic surgery in patients who had varus osteoarthritic knee and were treated with OWHTO combined with arthroscopic surgery. Among the 98 knees (88 patients) who underwent OWHTO between January 2008 and March 2013, 79 knees (71 patients) with more than 2 years of follow-up were reviewed retrospectively. The patients were divided into two groups: Group 1 (24 knees) underwent only OWHTO and Group 2 (55 knees) underwent OWHTO combined with arthroscopic surgery. For clinical evaluation, the range of motion (ROM), pain visual analog scale, Knee Society knee score, Knee Society function score, and complication were used. For radiologic evaluation, Kellgren-Lawrence grade, mechanical femorotibial angle, and posterior tibial slope were used. The average follow-up period was 29.1 months. Group 2 showed a significant increase in the ROM at the last follow-up (133.2 ± 6.0 degrees) compared with the preoperative time point (128.3 ± 7.7 degrees) (p < 0.001). In the comparison of radiologic parameters between Groups 1 and 2, there was no significant difference. In the minimum 24-month follow-up, when OWHTO combined with arthroscopic surgery was performed, arthroscopic surgery helped increase the ROM of patients with mechanical symptoms. However, the amount of the ROM increase of 4.9 degrees was of unknown clinical significance.