The aim of this study was to perform an external validation of O-RADS risk stratification to differentiate between benign and malignant adnexal masses. This is a retrospective diagnostic accuracy study of data collected from patients with adnexal pathology who underwent transvaginal ultrasound, performed by gynecologists or radiologists prior to surgery. The study was conducted in a tertiary university oncology hospital between 2012 and 2020. All reports used the ovarian-adnexal reporting and data system by O-RADS group. Definitive pathology after tumour surgical removal was the reference standard used in this study. Sensitivity, specificity, positive (PPV) and negative predictive values (NPV) were calculated. A total of 746 patients were examined and reported. 29 women were excluded as they were not taken into surgery. A total of 717 women, 520 premenopausal and 197 postmenopausal, were included. The pathology report revealed 77 malignant and 640 benign tumours. O-RADS had a sensitivity, specificity, PPV and values NPV of 90%, 86%, 45% and 98%, respectively. In this external validation study, the O-RADS risk stratification tool showed a high diagnostic performance for the differentiation between benign and malignant adnexal masses.