WASHINGTON (Reuters) - U.S. President Donald Trump picked U.S. hostage negotiator Robert O’Brien on Wednesday as his fourth White House national security adviser, turning to a low-key choice for the position after the boisterous tenure of John Bolton.

O’Brien’s selection was a sign of U.S. Secretary of State Mike Pompeo’s clout with the president, with U.S. officials saying Pompeo had made clear he would be happy with either O’Brien or another candidate, former deputy national security adviser Ricky Waddell.

“I have worked long & hard with Robert. He will do a great job!” Trump said in a tweet.

Aides said Trump had gotten to know O’Brien through his work as the U.S. envoy for hostage negotiations and admired his ability to get hostages returned from North Korea and Turkey.

The job most recently took him to Sweden in a bid to get the American rapper known professionally as A$AP Rocky out of jail on an assault charge.

Trump appeared before reporters with O’Brien at his side on the tarmac alongside Air Force One in Los Angeles.

“I think we have a very good chemistry together and I think we’re going to have a great relationship. He is a very talented man,” Trump said.

A source close to the White House said Trump wanted to pick a new adviser who would be able to get along well with Pompeo after the secretary of state sometimes struggled with Bolton.

O’Brien, the source said, “is a low-profile, articulate negotiator who has a strong relationship with Pompeo.”

O’Brien follows in the footsteps of three other national security advisers: Michael Flynn and H.R. McMaster and, most recently, Bolton, who clashed with the president over a host of issues from Iran to Afghanistan to North Korea.

Bolton parted ways with Trump a little more than a week ago, his stormy ending coming shortly after he disagreed with the prospect of the president easing some sanctions on Iran, a person close to Bolton said.

Within the National Security Council, O’Brien’s critical task will be to stabilize a sprawling foreign policy apparatus where morale has taken a major hit since Trump took office, according to an NSC insider.

Trump’s abrupt firing of Bolton added to a sense of unease among NSC staff. 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.

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For Trump, O’Brien represents a toned-down choice for the job, a sign that the president was happy to have someone without the television starpower of Bolton.

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. He has handled a number of high-profile legal cases and previously served in several State Department positions, including as an alternative representative to the U.N. General Assembly in 2005.

O’Brien has been a fan of the recently departed Bolton since the two worked together when Bolton was U.S. ambassador to the United Nations in 2005 during George W. Bush’s presidency.

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In a December 2016 column for radio host Hugh Hewitt’s website, O’Brien called Bolton “a formidable diplomat and a patriot” in recommending the newly elected Trump pick Bolton for a high-profile assignment.

“John’s job as our man at the UN was never easy, often exhausting and painfully slow at points. But John, the definition of a diplomat, never grew physically tired or ever lost his temper with other diplomats or the mission’s staff,” O’Brien wrote.

Trump in March had complimented O’Brien for doing a “fantastic job” after gaining the release of American hostage Danny Burch in Yemen.

People close to the White House said Trump was looking for someone who would manage the national security process, voice opinions behind the scenes but not go public with differences.

One of the most prominent hostage cases O’Brien has worked on is that of Austin Tice, a freelance journalist believed by the U.S. government to be alive who was abducted in Syria in August 2012 while reporting on the uprising against President Bashar al-Assad.

Tice’s parents, Marc and Debra Tice, praised what they described as O’Brien’s quiet, dogged efforts to win their son’s release.

“He’s kept us informed and been regularly in touch,” Marc Tice told Reuters in a telephone interview from his Houston home. “With Robert in this new role, Austin’s return can happen even sooner.”