O'Brien's appointment comes one week after Bolton submitted his resignation to the President.
"I have worked long and hard with Robert", Trump tweeted in his announcement of the replacement. "He will do a great job!", he added.
Mr. Trump also liked O'Brien because he "looked the part" of national security adviser, sources told CBS News.
O'Brien has been a fan of the recently departed Bolton after the two worked together when Bolton was USA ambassador to the United Nations in 2005 during George W. Bush's presidency.
O'Brien, U.S. Special Envoy Ambassador, centre, arrives at the district court where United States rapper A$AP Rocky is to appear on charges of assault, in Stockholm, Sweden, Tuesday July 30, 2019. He works closely with the families of American hostages and advises the senior leadership of the U.S. Government on hostage issues.
His hostage role had been overseen by Mike Pompeo, the U.S. secretary of state, whose power within the administration is growing.
Robert O'Brien, former US special envoy ambassador, arrives at the district court in Sweden to follow the trial against American rapper A$AP Rocky, on August 1.
"Close to the president and the president likes him is a qualification for a lot of positions in the administration", said Swerdlick. Previously, he worked for Bolton when Bolton was United Nations ambassador in the George W. Bush administration.
Unlike other high-level cabinet positions, Trump's selection does not require Senate confirmation. Given Trump's promise on Wednesday to "substantially increase" sanctions on Tehran, O'Brien's hiring likely signals no thaw in relations with the Islamic Republic.
However, Mr O'Brien also has long links to Mr Bolton.
He also was an adviser on the Republican presidential campaigns of former Governor Scott Walker of Wisconsin, former Governor Mitt Romney of MA and Senator Ted Cruz of Texas. If recent history is any guide, Trump sees himself as his own national security adviser; he does not read intelligence reports; and he has little use for the conclusions of his own country's intelligence agencies. O'Brien was a Major in the U.S. Army Reserve. Trump said last week that he'd asked his chief diplomat if he was interested in also taking on the national security adviser job but that he had declined. 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.
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