He was chosen to portray legendary lawman Wyatt Earp on the ABC Western series "The Life and Legend of Wyatt Earp", which debuted in 1955. To help develop his character, O'Brian bought Stuart N. Lake's book '. He also developed a relationship with Lake, who was a consultant on the show for the first two years. The series, alongside "Gunsmoke" and "Cheyenne", which debuted the same year, spearheaded the "adult Western" television genre, with the emphasis on character development rather than moral sermonizing. It soon became one of the top-rated shows on television. During its six-year run, "Wyatt Earp" consistently placed in the top 10 in the United States. Decades later, he reprised the role in two episodes of the television series "Guns of Paradise" (1990), the television movie ' (1991), and the independent film "" (1994), the latter mixing new footage and colorized archival sequences from the original series.