Castor Oyl is Olive Oyl's older brother. He was the main protagonist of Thimble Theater for years before the first appearance of the Popeye character in January 1929. A quick-witted yet diminutive adventurer, Castor Oyl continued to be an important character in Segar's Popeye strip, but played little (most usually no) role in the Popeye theatrical cartoons produced first by Fleischer Studios and then by Famous Studios. His most prominent appearance in the series is as a member of Popeye's orchestra in the 1935 short The Spinach Overture.
In the 1980 film directed by Robert Altman, Castor is a key character, and is played by actor Donovan Scott. In the film, however, he is made Olive's younger brother and is considerably less sophisticated than his comic strip counterpart.
Castor Oyl is named after castor oil, a medicine often given to children in the 1920s and 1930s. His mother Nana's name derives from "banana oil," an epithet similar to "baloney." His father Cole's name derives from "coal oil," a now-obsolete term for kerosene.