Purpose: The purposes of this study are three-fold: (1) To examine whether the WOMAC questionnaire should be obtained before or after performance-based tests. (2) To assess whether self-reported disability scores before and after performance-based tests differ between obese and non-obese individuals. (3) To observe whether physical activity and BMI predict self-reported disability before and after performance based tests. Methods: A longitudinal study included thirty one participants diagnosed with knee osteoarthritis (OA) using the Kellgren-Lawrence Scale by an orthopedic surgeon. Results: All WOMAC scores were significantly higher after as compared to before the completion of performance-based tests. This pattern of results suggested that the WOMAC questionnaire should be administered to individuals with OA after performance-based tests. The obese OA was significantly different compared to the non-obese OA group on all WOMAC scores. Physical activity and BMI explained a significant proportion of variance of self-reported disability. Conclusion: Obese individuals with knee OA may over-estimate their ability to perform physical activities, and may under-estimate their level of disability compared to non-obese individuals with knee OA. In addition, self-reported physical activity seems to be a strong indicator of disability in individuals with knee OA, particularly for individuals with a sedentary life style.