President Donald Trump on Wednesday, Sept. 18, named Robert O'Brien, who has served as the U.S. hostage negotiator, as his new national security adviser, replacing the ousted John Bolton.

"I am pleased to announce that I will name Robert C. O'Brien, currently serving as the very successful Special Presidential Envoy for Hostage Affairs at the State Department, as our new National Security Adviser," Trump said in a tweet. "I have worked long & hard with Robert. He will do a great job!"

O'Brien, who becomes Trump's fourth national security adviser, was boosted by Secretary of State Mike Pompeo, with whom he had worked on a number of hostage cases.

Inside the Trump administration, O'Brien was viewed as the "safest option" at a time when the national security team wanted as little "drama" as possible going into the 2020 elections, said a senior U.S. official, who requested anonymity to speak more candidly.

"He gets along with everybody," the official said. "He's the nicest guy on the planet."

His friendly demeanor contrasts with that of his predecessor Bolton, who rankled officials at the Pentagon and the State Department with his sharp-elbowed management style and revamp of the policy process that involved fewer meetings for senior officials to air their views.

Officials said a policy process that doesn't create new competing factions would be welcome, particularly by acting White House chief of staff Mick Mulvaney.

Upon taking the job, O'Brien will become the highest-ranking Mormon in the U.S. government, an important milestone for a religious community that has shown some skepticism of Trump and will be an important voting demographic in certain states, particularly Arizona.

O'Brien was a founding partner of a Los Angeles law firm and has also served in U.S. government roles focusing on Afghanistan and the Middle East.

O'Brien has praised Trump for having "unparalleled success" in bringing home hostages, though his appearance in Stockholm in July to monitor the trial of U.S. rapper A$AP Rocky raised eyebrows as critics assailed Trump for what they viewed as an inappropriate intervention in an allied nation's legal matters.

The announcement of O'Brien came a day after Trump publicly named five finalists for the position, none of whom were well-known.

As he flew to California for a political fundraising swing on Tuesday, Trump said others on his shortlist were Army Maj. Gen. Ricky Waddell, the assistant to the chairman of the Joint Chiefs of Staff, who also has served as deputy national security adviser; Lisa E. Gordon-Hagerty, Energy Department undersecretary for nuclear security; former Bolton chief of staff Fred Fleitz; and retired Army Lt. Gen. Keith Kellogg, who serves as the national security adviser to Vice President Mike Pence.

Bolton was ousted last week, ending a stormy tenure marked by widening rifts between an unorthodox president seeking a foreign policy victory and an irascible foreign policy hawk who had been deeply skeptical of much of the president's agenda.

Bolton's opposition to elements of Trump's approach on North Korea, Iran and Afghanistan, among other issues, put him at odds with his boss and other advisers. Trump also largely blamed his third national security adviser for overselling the strength of Venezuela's political opposition earlier this year.

- - -

This article was written by John Wagner and John Hudson, reporters for The Washington Post.

The Washington Post's Anne Gearan contributed to this report.