Dennis Joseph O'Toole (born March 13, 1949) is an American former professional baseball player.

A right-handed relief pitcher and the younger brother (by 12 years) of 1963 National League All-Star lefthander Jim O'Toole, Dennis appeared in 15 Major League games spread over five seasons (1969–1973) for the Chicago White Sox.

In 30⅓ MLB innings pitched, O'Toole gave up 43 hits and ten bases on balls, with 22 strikeouts and no saves.

He compiled a 53–39 (3.66) mark in 211 minor league games, mostly in the White Sox' farm system, over nine seasons (1967–1975).

[1] His MLB career did not overlap with Jim's, who hurled his last big league game with the ChiSox over two years before Dennis' debut.