(March 31, 1917 - 1930) (separated) (2 children)
O'Brien was married to Hazel Ruth Collette in 1917 and they had two sons together, William and Willis, Jr., but the marriage was an unhappy one, which O'Brien was reportedly forced into and rebelled against with drinking, gambling, and extra-marital affairs. The couple had divorced by 1930 and the two boys remained with their mother, who had begun to show unbalanced behaviour. By 1931 Hazel had been diagnosed with cancer and tuberculosis, while William also contracted tuberculosis resulting in blindness in one eye and then the other.
The success of King Kong led to the studio commissioning a hurried sequel Son of Kong (also 1933), which O'Brien described as cheesy. With a limited budget and a short production schedule O'Brien chose to leave the animation work to his animation assistant, Buzz Gibson, and asked the studio not to credit him on the project. While making one of his daily visits to the set, O'Brien, who had remained close to his two sons after his separation from his estranged wife, invited Willis Jr. and the now completely blind William with him to handle the Kong and dinosaur models. A few weeks after this visit O'Brien's ex-wife, Hazel Ruth Collette, shot and killed William and Willis Jr. before turning the gun on herself. The suicide attempt failed and by draining her tubercular lung actually extended her life by another year. A publicity photo of O'Brien taken around this time shows the anguish on his face. Hazel Ruth Collette remained in the Los Angeles General Hospital prison ward until her death in 1934.