Here is an interesting thing I read about the correlation between Osteoarthritis and Vitamin D. According to one Australian study of 880 people between the ages of 51 to 79, they noticed upon examination of their knees that the ones deficient in Vitamin D had significantly less knee cartilage and more pain than others suffering from the same condition.
Though Vitamin D will not reverse the disease, it can help to slow the progression and may put off the need for surgery, those researchers had found.
Since few foods are high in Vitamin D, it is better to take a supplement to safeguard yourself. The recommended requirement currently suggests 1,000-2,000 IUs. However, that dosage is set to change and will probably increase. Dosage may vary from one individual to another depending on age, weight, other supplement, herbs or drugs that you're taking, etc.
However, it you intend to go beyond 4,000 IUs a day, you may need a blood test to see if you're not getting too much and harming yourself. Do check with your doctor to find the exact needed amount that's safe and right for your own body.