Introduction and hypothesis Obstructive sleep apnea syndrome is associated with urological symptoms, including overactive bladder (OAB). This study aims to determine whether combined tolterodine and CPAP therapies are more effective for patients with OSAS than CPAP treatment only. Methods Women who underwent polysomnography test and were diagnosed with moderate-to-severe OSAS with apnea-hypopnea index (AHI) were included in the study. Data were collected on AHI, OAB awareness-8-item tool (OAB-V8), incontinence questionnaire-urinary incontinence short form (ICIQ-UI-SF), total daily urine volume (DUV), and the Benefit, satisfaction with treatment and willingness (BSW) tool. Eligible patients were randomized to receive either CPAP treatment only or combined CPAP and tolterodine treatment for 3 months. Results Among 103 participants, a total of 60 were included. Patients in both treatment arms showed significant improvements in OAB-V8, ICIQ-UI-SF, and total DUV compared to their baseline. The mean OAB-V8 was 15.7 at baseline and 5.6 at 3 months for the combined treatment arm and 16.6 and 7.6 at 3 months for the CPAP group only (mean baseline-adjusted between-group difference -1.1 [95% CI, -12.3 to -7.4]; p < 0.001). The improvement in the mean ICIQ-UI-SF was also statistically more significant in the combined therapy group than in the CPAP only arm (mean baseline-adjusted between-group difference -3.27 [95% CI, -4.6 to -1.59]; p < 0.001). No statistical significance was found in the improvement of total DUV between the groups. Conclusions In this study, combined use of tolterodine with CPAP provides beneficial effects to CPAP treatment only regarding OAB symptoms. Further research is required to confirm these findings in a large cohort.