He impressed the president in that job by securing the release of several Americans imprisoned by foreign governments and armed groups. Mr. Trump views the release of detained Americans as tactical “wins” that even his critics are reluctant to question, and Mr. O’Brien continues to pursue those cases in his new job.
Some White House aides joke that his experience navigating fraught situations is ideal preparation for serving Mr. Trump. The president, for his part, appreciates Mr. O’Brien’s quiet manner and tailored suits after his complaints about the gruff personalities and unstylish appearances of Mr. McMaster and Mr. Bolton, whose bushy mustache he often privately mocked.
Mr. O’Brien also gets along better than his predecessors did with Mr. Pompeo, who feuded with Mr. Bolton, and with Mr. Trump’s son-in-law, Jared Kushner, who often gives foreign policy advice. And he has been friends with Mr. Grenell, the latest addition to the national security team, for over a decade.
He speaks with the president several times a day, often first thing in the morning and sometimes in the White House’s private residence before Mr. Trump descends to the Oval Office.
But he has no prior ties to Mr. Trump or previously known affinity for the president’s “America First” style of nationalism. A book of essays he published in 2016 channeled mainstream conservative views.
Some national security professionals who have worked with or advised Mr. O’Brien say that it is a mistake to underestimate him and that he has a deft managerial touch that reflects his tenure leading dozens of lawyers in the Los Angeles office of Arent Fox, the Washington law firm.
Others complain that he lacks fluency in policy details and delegates heavy lifting to his chief deputy, Matthew Pottinger, a former Wall Street Journal reporter and Marine who is among a handful of White House aides to survive all three years of Mr. Trump’s presidency.
In a television interview in late December, Mr. O’Brien incorrectly referred to the North Korean ruler, Kim Jong-un, swapping the leader’s surname for his given one. It was, his critics said, not a mistake that a more experienced official would have made.
Julian E. Barnes, Adam Goldman, Katie Rogers and Edward Wong contributed reporting.