What are the characteristic X-ray features of osteoarthritis?

This is a classic question, and a useful answer to memorise. The characteristic changes of osteoarthritis (OA) are:

• Narrowing of joint space - The 'space' between most joints that you can see on X-rays is due to the intervening cartilage not showing up. In OA, the progressive loss of cartilage thickness that occurs causes the joint space to appear narrowed.


• Subchondral sclerosis - probably as a compensatory reaction to the increased stresses transmitted to the more exposed bone, the bone under the defective cartilage becomes thicker and harder (i.e. sclerotic)


• Subchondral cyst formation - The traumatised bone underlying the affected cartilage sometimes displays areas of cystic degeneration, forming fluid-filled 'sacs'.
  


• Osteophyte formation - As the disease progresses, cartilage in the peripheral unstressed areas proliferates and ossifies. The resultant bony outgrowths are called osteophytes, and it is believed that they function to improve the joint space congruence. This interpretation is controversial, however, and the osteophytes are often a major cause of pain.

The image below (of the knee joint) shows all the above apart from cyst formation. The black arrows point to subchondral sclerosis, the white arrow points to a small osteophyte and the black arrowheads indicate the joint space narrowing. The image is from the Atlas of Radiological Images, contained in the "Harrison's e supplement".

There's also a nice diagram of the changes here.