OSI Pharmaceuticals, Inc. is an American pharmaceutical company based in Long Island, New York with facilities in Colorado, New Jersey and the United Kingdom. OSI specializes in the discovery and development of molecular targeted therapies, and was listed in the NASDAQ Biotechnology Index. Though oncology is the top priority for OSI, research and development targeting type 2 diabetes and obesity is conducted through their U.K. subsidiary Prosidion Limited. OSI has also made a foray into the ophthalmology market through a marketing agreement with Pfizer over Macugen (Pegaptanib) for Age-related macular degeneration; however, acquisition of the firm Eyetech, meant to provide control over this product and diversify the company, has been unsuccessful, ending in divestiture.

As of mid-2007, OSI's revenues are based primarily on proceeds from Tarceva sales (which are shared with Genentech and Hoffmann–La Roche) and royalty payments related to dipeptidyl-peptidase IV inhibitor intellectual property.