There are two different types of OA

Primary

This type of OA is a chronic degenerative disorder related to but not caused by aging, as there are people well into their nineties who have no clinical or functional signs of the disease.

As a person ages, the water content of the cartilage decreases due to a reduced proteoglycan content, thus causing the cartilage to be less resilient. Without the protective effects of the proteoglycans, the collagen fibers of the cartilage can become susceptible to degradation and thus exacerbate the degeneration.

Inflammation of the surrounding joint capsule can also occur, though often mild (compared to that which occurs in rheumatoid arthritis). This can happen as breakdown products from the cartilage are released into the synovial space, and the cells lining the joint attempt to remove them. New bone outgrowths, called "spurs" or osteophytes, can form on the margins of the joints, possibly in an attempt to improve the smoothness of the articular cartilage surfaces. These bone changes, together with the inflammation, can be both painful and debilitating.

Secondary

This type of OA is caused by other factors or diseases but the resulting pathology is the same as for primary OA:

Congenital disorders, such as:
Congenital hip luxation
People with abnormally-formed joints (e.g. hip dysplasia) are more vulnerable to OA, as added stress is specifically placed on the joints whenever they move. [However, recent studies have shown that 'double-jointedness' may actually protect the fingers and hand from osteoarthritis.]
Cracking joints—the evidence is weak at best that this has any connection to arthritis.
Diabetes.
Inflammatory diseases (such as Perthes' disease), (Lyme disease), and all chronic forms of arthritis (e.g. costochondritis, gout, and rheumatoid arthritis). In gout, uric acid crystals cause the cartilage to degenerate at a faster pace.
Injury to joints, as a result of an accident.
A joint infection, e.g. from an injury.
Hormonal disorders.
Ligamentous deterioration or instability may be a factor.
Obesity. Obesity puts added weight on the joints, especially the knees.
Osteopetrosis (High bone density).
Sports injuries, or similar injuries from exercise or work. Certain sports, such as running or football, put undue pressure on the knee joints. Injuries resulting in broken ligaments can lead to instability of the joint and over time to wear on the cartilage and eventually osteoarthritis.
Pregnancy