Study Objective This study compares performance of O-RADS (version 2022) and Simple Rules (SR) ultrasound criteria in a cohort of asymptomatic pathology-proven adnexal masses and evaluates O-RADS and SR inter-observer agreement in a subset of patients. Design Retrospective Cohort Study. Setting Diagnostic imaging. Patients or Participants Consecutive women who underwent surgical resection of adnexal mass(es) between January 2008 and December 2018 at two University Hospitals, a time period when cine clips were available for all ultrasounds. Interventions One experienced radiologist, blinded to pathological diagnosis, categorized all imaging by O-RADS and SR criteria. Measurements and Main Results 791 adnexal masses in 762 patients were assessed, aged 18-92 (44 ± 15); 628 benign, 49 LMP, 114 malignant. O-RADS categories were 2 (n=309), 3 (n=165), 4 (n=181), 5 (n=136) with malignant rates of 0.3%, 3%, 25%, and 82% respectively. Application of simple rules criteria identified 561 masses as benign and 230 as malignant. Combining O-RADS 4 and 5 categories as being malignant, sensitivity, specificity, NPV, PPV, and accuracy to detect invasive/LMP masses were 96% (CI:92-99%), 75% (CI:71-78%), 99% (CI:97-100%), 49% (CI:44-55%), and 79% (CI:76-82%). Corresponding results for SR were 96% (CI:91-98%), 89% (CI:85-91%), 99% (CI:98-100%), 68% (CI:61-74%), and 90% (CI:87-92%) with specificity, PPV, accuracy of SR being statistically significantly higher than O-RADs (p<0.0001). AUC-ROC of SR and O-RADS were 0.920 and 0.855 (p=0.01). Inter-observer agreement between the three readers for review of a subset of 172 masses were 0.89, 0.91, and 0.93 for SR benign vs malignant and 0.71, 0.75, and 0.75 for O-RADS (2/3) vs O-RADS 4/5. Conclusion Adnexal mass assessment with SR performs significantly better than US O-RADS classification in specificity, PPV, and accuracy. Risk stratification by experienced radiologists using SR criteria outperforms O-RADS and can result in better triage to surgical gynecologists and oncologists with an improved rate of predicting malignancy.