The first example of anti-Ok^a was found in a Japanese woman and was identified by Morel and Hamilton. Since she had never been pregnant, the antibody most likely was the result of a prior blood transfusion. Two of her three siblings were also found to be Ok(a-). The Ok(a-) phenotype appears to be limited to the Japanese as the only other (two) donors who were Ok(a-) were also Japanese.

A monoclonal anti-Ok^a was produced following immunization with cells from a teratocarcinoma. This was used to study the biochemical properties of the Ok^a antigen. The antigen is expressed on a glycoprotein having molecular weights ranging from 35,000-68,000. The gene for Ok^a is located on chromosome 19p13.2, however, its function is unknown.