Objective <p>To evaluate the diagnostic performance and inter-rater reliability of (O-RADS) MRI based on non-dynamic contrast-enhanced (DCE) MRI and the value of apparent diffusion coefficient (ADC) in the characterization of adnexal masses.</p> Methods <p>Retrospective analysis was done for 148 patients with 191 adnexal masses who underwent non-DCE MRI and diffusion-weighted imaging (DWI). Two independent radiologists classified the masses into five categories according to O-RADS-MRI and measured ADCmean values for solid and cystic components. The final diagnoses were determined by postoperative histopathology. Logistic regression analysis was performed to detect predictors of malignancy.</p> Results <p>There was excellent inter-rater agreement in assessing O-RADS categories (k_w=0.901 and 95% CI = 0.863 to 0.939). Solid tissue enhancement more than the myometrium at 30–40&#xa0;s at non-DCE MRI was statistically significantly higher in malignant lesions (<i>p</i> &lt; 0.001). There was also excellent reliability (absolute inter-rater agreement) in measuring ADCmean values. The ADCmean of solid and cystic components at cutoff values of ≤ 1.3 × 10 − 3 mm2/s and &gt; 2.07 × 10 –3&#xa0;mm²/s are perfect and excellent discriminators for differentiating malignant and benign adnexal masses (AUC = 1.000 and 0.952) respectively. Multivariate regression analysis revealed that the presence of simple fluid, low DW signal of cystic component, and ADCmean of cystic component &gt; 2.07 × 10 –3&#xa0;mm²/s were statistically significant independent predictors of malignancy in an O-RADS-MRI score &gt; 3 adnexal lesions.</p> Conclusions <p>O-RADS-MRI score utilizing non-DCE MRI is a reliable system with high PPV. ADCmean value is a non-invasive excellent discriminator for differentiating adnexal lesions that could improve O-RADS-MRI score performance and treatment plan.</p>