“I have worked long & hard with Robert. He will do a great job!” Trump wrote on Twitter.
In his current role, O’Brien “leads the U.S. government’s diplomatic efforts on overseas hostage-related matters,” according to his biography on the State Department’s website.
O’Brien was involved in the release of US.. rapper ASAP Rocky by Swedish authorities in August, after the musician had been detained in connection with a street brawl in Stockholm.
The special envoy was in the courtroom during the trial, which stirred diplomatic tensions as Trump demanded the rapper’s release.
O’Brien previously co-chaired the State Department’s justice reform initiative in Afghanistan, as well as advised the government on the trafficking of antiquities, according to his biography.
He worked with Bolton at the United Nations under President George W. Bush, who appointed him in 2005.
O’Brien takes over as Trump’s fourth national security adviser, following the departures of Bolton, Michael Flynn and HR McMaster.
Trump said last week that he had asked Bolton to resign, citing strong differences of opinion with his hawkish assistant in the latest shake-up of an already turbulent White House.
Bolton’s ouster came as a surprise, with the White House putting out an advisory just an hour before Trump’s announcement, saying that Bolton would be taking part in a media briefing with Secretary of State Mike Pompeo.
Bolton also continued to post administration policy on social media right up until Trump’s announcement.
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.
Bolton appeared to be waging numerous battles within the White House, most recently over Afghanistan and North Korea, that seemed at odds with the president’s wishes.
He was also behind the hard-line approach to countries such as Iran and Venezuela.
Bolton’s deputy, Charlie Kupperman, stepped into the role until the appointment of a new security adviser. (dpa/NAN)