And even as Trump's transition was racked by chaos in a variety of areas, O'Brien proclaimed, "We may be witnessing the most impressive presidential transition from a national-security standpoint in history". In the most unusual episode, Mr O'Brien was dispatched to United States ally Sweden to attend the trial of USA rapper ASAP Rocky, who was accused of assault.
In terms of his hostage work for the State Department, O'Brien wrote in an article in late 2018 that Iran is the "worst kidnapper of Americans". He also worked at the United Nations with Bolton.
"He's a very sharp guy", said Kent Lucken, who was a colleague of O'Brien's on two Romney campaigns. Mr Trump tweeted as he announced the decision.
O'Brien replaces John Bolton, the hawkish interventionist sacked last week. Earlier in his career, O' Brien was a senior legal officer for the UN Security Council commission that decided claims against Iraq that arose from the Gulf War.
He has also been the only USA official consistently working on releasing Americans imprisoned in Iran.
Trump's first choice, Michael Flynn, was gone after three weeks for lying to the administration - and subsequently, the Federal Bureau of Investigation - over contacts with Russian Federation during the Trump transition to government. Bolton's departure left Pompeo unchallenged as Trump's closest foreign policy adviser. O' Brien has a law degree from the University of California, Berkeley, and co-founded a law firm in Los Angeles focused on litigation and global arbitration issues.