Diomedes Antonio Olivo Maldonado (January 22, 1919 – February 15, 1977), nicknamed "Guayubin" for his hometown, was a Dominican professional baseball player and scout. The left-handed pitcher appeared in 85 Major League Baseball pitched over all or part of three seasons between 1960 and 1963 for the Pittsburgh Pirates and St. Louis Cardinals. Prior to his minor league and Major League career, Olivo spent many years playing in his native Dominican Republic. He was the brother of fellow major leaguer Chi-Chi Olivo, and the father of major league pitcher Gilberto Rondón.
Diomedes Olivo posted a 55–29 won-loss record in the Double-A Mexican League from 1955–1959, then made his Major League debut with the Pirates at age 41 on September 5, 1960. His age at the time of his MLB debut is the oldest with the exception of Satchel Paige in the post-World War II era. In his first game, he pitched two scoreless innings of relief against the Milwaukee Braves, allowing one hit (to eventual Hall of Famer Eddie Mathews) and two bases on balls.
Olivo's best MLB season came in 1962. At age 43, he worked in 62 games, all but one in relief, and posted a 5–1 record and 2.77 earned run average in 84⅓ innings pitched, with seven saves. He then was included, with shortstop Dick Groat, in a major off-season trade to the Cardinals, but while Groat sparkled in St. Louis, Olivo lost all five decisions and spent part of 1963 in the minor leagues. He retired at age 44 following that season.
During his MLB career, Olivo allowed 112 hits and 39 bases on balls in 107⅓ innings pitched, striking out 85.
He scouted for the Cardinals after retiring from the field, and later held a position in the Ministry of Sports in his native country until his death, from a heart attack, at age 58.