In 1931, along with Francis Beresford-Hope and Henrik Mellberg, he recovered an artifact related to the Tomb of the Gods in a diamond mine near the Popigai Crater, north of Norilsk, Siberia. They formed a group of fifteen men to follow the map on the artifact and reached the Tomb of the Gods. Only O'Brien, Beresford-Hope and Mellberg returned from the expedition alive. The three discoverers split the key into three pieces, and each kept a piece, in order to keep the key from being used by the wrong people.
Around 1936, O'Brien stopped communicating with Mellberg, and Mellberg assumed that O'Brien and his piece of the key had been captured by the Ahnenerbe, the ancient antiquities branch of the SS. Fearing that the Nazis had O'Brien's piece, Mellberg contacted Indiana Jones to help prevent the Nazis from collecting all the pieces. Mellberg's assumption was correct as Friedrich von Hassell revealed that he had ended up with O'Brien's piece of the key.
O'Brien's fate is unknown.