President Trump's newly named national security adviser,, replaces John Bolton, who last week. Here's what to know about O'Brien, the fourth person Mr. Trump has named to the position since he took office:
O'Brien is currently the State Department's special presidential envoy for hostage affairs. O'Brien was a key player in negotiating the release of Danny Burch, an American who was imprisoned in Yemen, earlier this year. O'Brienin the White House with Mr. Trump and Vice President Mike Pence in March.
He has also been the only U.S. official consistently working on releasing Americans imprisoned in Iran. The position is a relatively recent one, created during the Obama administration after it had been widely criticized by families of the ISIS hostages for its handling of their cases. It's a role that has been kept on by the Trump administration to continue to focus on efforts to bring Americans home.
The family of Robert Levinson, a former FBI agent who disappeared in Iran in 2007, told CBS News in a statement on Wednesday that they were "very glad to hear" of O'Brien's appointment, calling him a "strong advocate within the U.S. government for our father."
They called the appointment "further evidence of President Trump's commitment to bringing home Americans held abroad."
Earlier this year Levinson's wife told lawmakers during a hearing in the House of Representatives on Americans detained in Iran that she felt her husband had been "deprioritized, or seemingly forgotten," and she called for more efforts to secure his return.
O'Brien was also integral to thefrom a jail in Sweden. Rocky was accused of assaulting a man while in Stockholm earlier this year. O'Brien was present in court in Sweden when Rocky's trial began in early August.
In his capacity as special envoy for hostage affairs, O'Brien wrote a letter to Swedish prosecutors urging them to release Rocky.
"The government of the United States of America wants to resolve this case as soon as possible to avoid potentially negative consequences to the U.S.-Swedish bilateral relationship," O'Brien wrote, according to NBC News.
His work on Rocky's case endeared him to Jared Kushner, the president's son-in-law and one of his top advisers.
In the Bush administration
O'Brien had previously worked with Bolton during George W. Bush's administration. In 2005, he was nominated and confirmed to be the U.S. alternate representative to the United Nations, while Bolton was U.S. ambassador to the United Nations.
He's also the author of the 2016 book While America Slept: Restoring American Leadership to a World in Crisis. The book, according to a review that appeared in Foreign Policy, warns against a hollowed-out military and in particular, having a "numerically short navy," and discusses the threats posed by China and Islamic extremism.
Mr. Trump also liked O'Brien because he "looked the part" of national security adviser, sources told CBS News.
O'Brien, who received his bachelor's degree from the University of California, Los Angeles and his law degree from U.C. Berkeley, is the co-founding partner of Larson O'Brien LLP in Los Angeles. He also served in the Army Reserve Judge Advocate General's Corps.
Kathryn Watson, Margaret Brennan and Christina Ruffini contributed to this report