Background: The aim of this study was to evaluate the psychometric properties of the Brazilian version of the Oral Anticoagulation Knowledge (OAK) Test. Methods: This study, conducted in an anticoagulation clinic, included 201 Brazilian participants aged over 18 years, who had been using warfarin for more than two months. The reliability of the instrument was evaluated by assessing internal consistency (Kuder-Richardson coefficient) and reproducibility (test-retest reliability). The validity was evaluated by hypothesizing that there would be a positive correlation of moderate to strong intensity between the correctness levels of the OAK Test and time within therapeutic range (TTR) values, which is a measure used to evaluate the quality of oral anticoagulation. Results: The instrument exhibited good psychometric properties. The total a Kuder-Richardson coefficient value was 0. 818 and intraclass correlation coefficient was 0.967. The validity revealed a strong positive correlation between the values of the level of knowledge, as measured by the OAK Test and the TTR values (rs =0.780). Conclusion: The instrument proved to be a reliable and valid tool for evaluating the knowledge of Brazilian patients on oral anticoagulation therapy with warfarin. This instrument may be incorporated into the practice of health care for substantiating the structuring of educational activities to ensure the improvement of knowledge about the use of warfarin, thereby increasing the effectiveness and safety of treatment