O'Brien first worked on cattle farms when he was thirteen years old before going on to become a short lived boxer winning nine out of his ten fights. All through this he used to sculpt models a hobby that would thrust him into the film business.
In 1913 at the San Fransisco World fair he created a dinosaur and a caveman and with help froma local cameraman he aniamted the models into a 90 second test shot. An exhibitor called Herman Wobber saw this footage and comissioned O'Brien to make his first film called "The Dinosaur and the Missing Link: A Prehistoric Tradgedy" in 1915. This was immediatly picked up by Thomas Edison and he hired O'Brien to create several short films based on the Prehistoric theme. These were called R.F.D 10,000 B.C and Prehistoric Poultry which were both made in 1917. During his time working for the Edison Company O'Brien worked on other pieces namely "The Puzziling Billboard" and "Nappy's Nightmare" which both came out in in 1917. These were very important pieces as these were the first shorts to include both live action and stop motion together. These films are what secured his postion on "The Lost Worlds" creative team. Although he usually worked in clay the models in this film were made out of rubber over a metal armature to allow for a more realistic look.
It was so popular that a sequel was demanded called "So of Kong" however O'Brien said it was too cheesy and gave the animating work to his is assistant and asked not to be be credited.
O'Brien then started work at RKO and help create more films such as "The Lat Days of Pompeii" in 1935 and "The Dancing Pirate in 1936 the latter being his first film in Technicolour.
O'Brien was also the Technical Creator of "Mighty Joe Young" (1949) won an Academy Award for Best Visual Effects in 1950.
His final piece of aniamtion came in "Its a Mad, Mad, Mad, Mad World" in 1963. He did a small scene within the film of characters runnign from a fire however he died before the film was released.
Willis O'Brien was a very talented special effects and stop motion user and his films are timeless classics that although have been remade are still remembered for their quality and creative flair.