He spent his spare time sculpting and illustrating and his natural talent led to him being employed first as draftsman in an architect's office and then as a sports cartoonist for the "San Francisco Daily News". During this time he also became a professional boxer, winning his first nine bouts but retiring after an unsuccessful tenth. He subsequently worked for the railroad, first as a brakeman and later a surveyor, as a professional marble sculptor, and was assistant to the head architect of the 1915 San Francisco World's Fair, where some of his work was displayed. During this time he made models, including a dinosaur and a caveman, which he animated with the assistance of a local newsreel cameraman. San Francisco exhibitor Herman Wobber saw this 90-second test footage and commissioned O'Brien to make his first film, "" (1915) for a budget of $5,000.