O'Brien's appointment comes one week after Bolton submitted his resignation to the President.
O'Brien, who now serves as the special presidential envoy for hostage affairs at the State Department, has a long history in Republican foreign policy circles.
"I have worked long & hard with Robert", Trump tweeted. "He will do a great job!"
However, Trump had previously hinted at his disagreements with Bolton on foreign policy, and observers had regularly noted that the two made an odd couple, with the president more isolationist and the national security adviser an interventionist.
As the special presidential envoy for hostage affairs at the State Department, Robert O'Brien worked closely with the families of American hostages and advised administration officials on hostage issues. "Pompeo may not be dual-hatted as secretary of state and NSA, but he might as well be with this arrangement", a former national security council spokesman, Ned Price, said on Twitter. Mr Bolton was UN Ambassador to the UN at the time. In 2005, Bush nominated O'Brien to be U.S. Representative to the United Nations General Assembly, where he worked with Bolton. O' Brien was confirmed by the Senate. When it was noted that this was an unusual task for a hostage negotiator - given the rapper wasn't being held hostage - O'Brien defended Trump's decision to send him. A key question is whether O'Brien will reinstate the inter-agency coordinating role for which the NSC was originally conceived but which was largely put on ice during Bolton's tenure, the source said. It will be interesting to see how O'Brien works under Trump.
O'Brien is an attorney from Los Angeles who served as a foreign policy adviser to 2012 Republican presidential nominee Mitt Romney and 2016 candidate Scott Walker. Bolton's departure left Pompeo unchallenged as Trump's closest foreign policy adviser. O'Brien was a Major in the U.S. Army Reserve.
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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.