The proposed study design is a randomised, placebo-controlled, double-blind clinical trial. We will recruit 400 subjects (50-79 years old, having relatively good health and serum vitamin D level of <60 and >12.5 nmol/L) with symptomatic knee OA for at least 6 months using a combined strategy in Southern Tasmania and Melbourne. Participants in the intervention arm (n=200) will receive 50,000 IU (1.25 mg) cholecalciferol tablets given once monthly, whilst those in the control arm (n=200) will receive an identical inert placebo. All participants will be provided recommended standard of care. Knee structural changes including knee cartilage volume, cartilage defects, tibial bone area, bone marrow lesions, and meniscal pathology (assessed by MRI), and knee pain at baseline and 2 years later will be determined as outcome measures. Other explanatory factors, such as serum vitamin D levels, height, weight, physical activity, and smoking will also be determined through study period.