Osteoarthritis is the most common form of arthritis, and treatments for it are varied and all have a limited amount of success for some, while being greatly successful for others. However, that being said, the one treatment that has been proven nearly all of the time to improve joint function and restore mobility is exercise. Not only can it help you lose weight if you need to, but it can strengthen muscles - which takes weight bearing off the joints, increase your tendon elasticity - which helps you me more mobile, and it can significantly reduce pain in the afflicted joint by helping lubricate it.
An effective treatment for knee related pain and arthritis by using exercise is to swim. Swimming not only helps you by the resistance of water, it requires more muscles to be used while not putting a weight bearing load on your bones and joints. This makes swimming relatively painless for knee, ankle, hip, and back problems - most of which are arthritis or injury related. Treating joint injuries quickly and not letting them get worse is a great way to prevent arthritis development in them. Taking a slow and paced walk can also improve mobility in the hips, knees, ankles, and back without straining the muscles or causing pain through over exertion.
Relaxation exercises, stretching, and gentle rotation of joints such as the shoulder and the hip can also improve mobility and reduce the effects of arthritic related pain, and are also a viable treatment option for those who suffer from mild pain in those joints. Hands and toes are also easily manipulated through simple but effective treatments that involve gentle stretching and movement until pain dissipates - along with strength building exercises - an example of this would be a simple construction involving a stick, a small high strength thin rope and a light weight that you could raise and lower slowly through the manipulation of the stick in your fingers (commonly called a hanging weight roll-up).