Osteoarthritis of the knee, a prevalent condition in older adults, can impact physical function and ability to perform physical activity. This randomized controlled trial examined the effects of a 6-month self-efficacy-based, individually delivered, lower-extremity exercise and fitness walking intervention with 6-month follow-up on physical activity and function. The 26 subjects were mostly older (M = 63.2 years, SD = 9.8), White (83%), obese (BMI M = 33.3, SD = 6.0) women (96%). Physical activity was measured by diaries. Physical function was measured by the 6-minute walk, Short Physical Performance Battery (SPPB), and WOMAC Physical Function subscale. Exercise self-efficacy was assessed by a questionnaire. Results showed significant increases in self-reported performance of lower-extremity exercise and participation in fitness walking distance in the 6-minute walk, and SPPB scores from baseline to 6-month follow-up with a trend for improvement in self-efficacy. Results suggest that the intervention was feasible, acceptable, and improved physical activity and function.