Oscar Davis Ratnoff (August 23, 1916 – May 20, 2008) was an American physician who conducted research on the process of coagulation and blood-related disorders.

Ratnoff discovered the substance later known as Factor XII and was one of the primary contributors to the delineation of the exact sequence that makes up the clotting cascade.

He also made notable research contributions to the understanding of the complement system and to the detection and treatment of hemophilia.

Ratnoff was a professor at the Case Western Reserve University School of Medicine, served as president of the American Society of Hematology, and was elected to the National Academy of Sciences.

Ratnoff survived and became a strong student at the Brooklyn Boys' School before enrolling at Columbia University when he was 16.

While a professor at Columbia, Patek had asked Ratnoff to review a research report on cirrhosis from noted physician Ernest Goodpasture.

The man had a long clotting time, but he had undergone successful surgery in the past without suffering from major bleeding.

Ratnoff had a long association with immunologist Irwin Lepow, and they conducted some of the early research on inhibition of the complement system.

[13] Ratnoff, who received his first National Institutes of Health (NIH) research grant in 1951, was still NIH-funded as an emeritus professor in the 1990s.