He began as a cowboy, then a marble-cutter, a newspaper cartoonist and even a prizefighter before discovering stop motion animation during his teens in 1914. Often featuring models of dinosaurs, O'Brien's efforts attracted the attention of the Thomas A. Edison, for whom he made ten five-minute shorts on Stone Age subjects.

After working for Edison's company, O'Brien worked for Herbert M. Dawley for The Ghost Of Slumber Mountain. O'Brien pioneered the use of rubber, rather than clay, models, an innovation that first reached the screen in 1925 when he worked on mixing live action and animation for First National's The Lost World in 1925, and his reputation was assured with his work on RKO's King Kong in 1933. During his career, there were many unrealised projects, including a version of Frankenstein as early as 1928. He died during production on It's A Mad, Mad, Mad, Mad World on November 8th 1962 at the age of 76.

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