The Osteoarthritis Initiative (OAI) is a longitudinal cohort study resource to investigate the natural history of knee osteoarthritis across the spectrum of the disease. The OAI includes clinical data and biospecimens. Data collected includes knee symptoms and function (measured using the West Ontario and McMaster Universities Arthritis Index (WOMAC) and the Knee injury and Osteoarthritis Outcome Score (KOOS)), hip and other joint symptoms, general quality of life, physical performance, knee examination, risk factors, health behaviors, psychosocial measures, and medication and supplements taken.

Biospecimens taken include blood, urine, DNA, and lymphocytes. Imaging data was also collected, including baseline and annual knee imaging (created using bilateral x-ray and bilateral knee MRI), as well as other joint imaging, including hip, pelvis, and hand x-rays, MRI's of the thigh, and full limb for knee alignment. Data was collected during a telephone screening at baseline, 12-, 24-, 36-, 48-, 72-, 96-month follow-up clinic visits, as well as during interim visits at 18 and 30 months, and questionnaires administered at 60-months, 84 months, and 108 months.