Objectives: Observer OPTION5 was designed as a more efficient version of OPTION12, the most commonly used measure of shared decision making (SDM). The current paper assesses the psychometric properties of OPTION5. Methods: Two raters used OPTION5 to rate recordings of clinical encounters from two previous patient decision aid (PDA) trials (n=201; n=110). A subsample was re-rated two weeks later. We assessed discriminative validity, inter-rater reliability, intra-rater reliability, and concurrent validity. Results: OPTION5 demonstrated discriminative validity, with increases in SDM between usual care and PDA arms. OPTION5 also demonstrated concurrent validity with OPTION12, r=0.61 (95%CI 0.54, 0.68) and intra-rater reliability, r=0.93 (0.83, 0.97). The mean difference in rater score was 8.89 (95% Credibility Interval, 7.5, 10.3), with intraclass correlation (ICC) of 0.67 (95% Credibility Interval, 0.51, 0.91) for the accuracy of rater scores and 0.70 (95% Credibility Interval, 0.56, 0.94) for the consistency of rater scores across encounters, indicating good inter-rater reliability. Raters reported lower cognitive burden when using OPTION5 compared to OPTION12. Conclusions: OPTION5 is a brief, theoretically grounded observer measure of SDM with promising psychometric properties in this sample and low burden on raters. Practice implications: OPTION5 has potential to provide reliable, valid assessment of SDM in clinical encounters.