Trivia

O'Brien's first marriage to Hazel Collette was a troubled and uneasy affair. By 1930 it had been dissolved, but not before having produced two sons, William and Willis Jr. Hazel suffered from tuberculosis and was often heavily sedated. The disease then spread to William and blinded him. In the fall of 1933, during the production of the film El hijo de Kong (1933), Hazel shot and killed her two sons and then turned the gun on herself. She survived the incident, but died from cancer and tuberculosis soon afterward.

After O'Brien's death his protege, Ray Harryhausen, made regular visits to his second wife, Darlyne, until her death.

On the day O'Brien received back the first printed footage of the film King Kong (1933) in motion, he noticed that the fur covering the King Kong puppet had moved because it was disturbed by his fingers during filming. He had to show the footage to the film's producer that day and was worried his boss would notice this supposed mistake and fire him. However, upon showing the producer this footage, the man applauded his ability and fine attention to detail, exclaiming that he was amazed that he had even managed to make Kong's fur blow around in the wind.

O'Brien spent significant pre-production time on various film projects that were never made; they included "Creation" (1931), "The War Eagles" (1938), "Gwangi" (1941) and "Emilio and Guloso" a.k.a. "Valley of the Mist" (1950).

In 1941 O'Brien wrote "Gwangi", the story that later became the film "The Beast of Hollow Mountain" (1956), and took it to Edward Nassour, who knew O'Brien as a teenager. Nassour bought the story inexpensively and promised O'Brien the job of animator. However, when production on the film was about to begin, O'Brien found that his story had been altered and he had been shut out of the studio. Nassour had hired someone else to do the film's special effects behind his back.

In 1928, O'Brien proposed a stop-motion animated version of "Frankenstein" and even shot test footage of it. Between 1958 and 1961, he proposed a stop-motion animated satirical second sequel to King Kong (1933) (the first sequel to it was El hijo de Kong (1933), which he personally disliked) titled "King Kong vs. Frankenstein," which was later changed to "King Kong vs. the Ginko" and then changed again to "King Kong vs. Prometheus." Neither one of these two Frankenstein films were ever made.

As a teenager, O'Brien tried to establish himself as a boxer and jockey.

Although O'Brien's father, William O'Brien, was self-educated and a noted etymologist, operated a military academy, and for 15 years served as assistant district attorney for Oakland, California, poor investments and bad luck forced the family into abject poverty when young Willis was 11 years old.