WASHINGTON – Secretary of State Mike Pompeo may have balked at the idea of becoming President Donald Trump's national security adviser, but many observers believe he'll still have a big say in how that job is executed moving forward.

Trump on Wednesday named State Department hostage negotiator and Pompeo ally Robert O'Brien to head up the the National Security Council, a sign to many that Pompeo's agenda will prevail after repeated clashes with former NSC head John Bolton, a foreign policy hardliner, over issues like Iran.

"Pompeo might have gotten the dual-hat, without the day job of pushing the paper at the NSC," said John Gans, author of "White House Warriors: How the National Security Council Transformed the American Way of War."

O'Brien is Trump’s special envoy for hostage affairs at the State Department. In that role, he has led the administration’s efforts to secure the release of Americans held by hostile foreign powers.

O'Brien will take the post at a turbulent moment in U.S. foreign policy. The Trump administration is considering a range of options to confront Iran over its alleged role in an attack on Saudi Arabia's oil fields. The president also wants to rekindle negotiations with North Korea over its nuclear arsenal and is engaged in a high-stakes trade war with China.

More:Trump says he will 'substantially increase' Iran sanctions over attack on Saudi Arabia oil facilities

“We’ve got a number of challenges," O'Brien told reporters after Trump introduced him as his new top national security aide during a stop in Los Angeles during a presidential fundraising tour.

Calling it a “privilege” to serve Trump, O'Brien praised the president and said his priorities are keeping Americans safe and rebuilding the U.S. military.

He declined to say what his advice would be on the current confrontation with Iran.

Praising his pick, Trump said he and O'Brien built "a tremendous track record on hostages," and that his new adviser is “highly respected."

“Robert has been fantastic," Trump said. "We know each other well.”

Trump said in April that O’Brien had called him the “greatest hostage negotiator” in U.S. history.

“President Donald Trump is the greatest hostage negotiator that I know of in the history of the United States. 20 hostages, many in impossible circumstances, have been released in last two years,” Trump tweeted at the time. The White House attributed the quote to O’Brien.

O’Brien has publicly praised Trump’s success in securing the release of American hostages, saying the president has had “unparalleled success” in freeing Americans.

Trump recently dispatched O’Brien to Sweden during the trial of rapper A$AP Rocky, a controversial decision given that the American music star was not a hostage but was accused of assaulting a man in Stockholm.

"I have worked long & hard with Robert. He will do a great job!" Trump wrote in a tweet announcing O'Brien's selection.

Bolton was dismissed as Trump's national security adviser Sept. 10. The president said the two "disagreed strongly" on foreign policy matters.

John Bolton:John Bolton gets back into political game after leaving White House

In addition to stylistic differences, Trump and Bolton clashed over U.S. policy toward Iran, Venezuela and North Korea. Bolton, a hawk, pushed Trump to take a hard line with those regimes and was deeply skeptical of negotiating with dictators.

Critics said the selection of the little-known O'Brien to such a high-profile post reflects a downgrade of the position and an increase in the influence of Pompeo, who was O'Brien's boss.

"Trump, after enduring the Bolton experience, clearly has no use for a strong personality or ideologue in the national security adviser position," said Ned Price, a spokesman for the National Security Council under President Barack Obama.

Price noted that Trump considered having Pompeo take over the national security adviser's job, in addition to his role as secretary of State. Pompeo didn't take Trump up on that offer, "but he might as well be (in both jobs) with this arrangement."

Gans and other experts said O'Brien may be seen as a cipher for Pompeo, and he could have a hard time exerting influence over the course of events, whether in Iran or other global hot spots.

"The challenge for now is that, with a number of issues breaking around the world, O’Brien is unlikely to have the strength to do the job Trump won’t do or get the president to do it," Gans said.

Aaron David Miller, a former State Department negotiator and senior fellow with the Carnegie Endowment for International Peace, said Pompeo wanted to "make sure there's no Bolton redux," and that he is "king of the hill" when it comes to foreign policy.

On the other hand, Miller noted that all advisers may have limited authority because Trump likes to make his own decisions.

"For better or for worse," Miller said, "Donald Trump is going to be his best – and worst – political and foreign policy adviser in the days ahead."

On the very day Trump announced O'Brien's appointment, some analysts noted, Trump took a low-key approach to questions about responding to Iran; Pompeo, meanwhile, described the attack on the Saudi oil facilities as "an act of war."

"Good luck in your new role coordinating the statements and actions of President Trump’s national security cabinet, Mr. O’Brien," said Heather Hurlburt, who directs the New Models of Policy Change project at the think tank New America.

Others said O'Brien brings unique strengths to the job.

"Robert O'Brien has the right background at this moment of crisis in the Middle East and beyond," said Notre Dame law school professor Mary Ellen O’Connell, who specializes in international law. "He is deeply immersed in the operation and promotion of the law that keeps the peace – a welcome change from the law-deriding, bellicose John Bolton."

Sen. Lindsey Graham, R-S.C., a defense hawk often allied with Bolton, called O'Brien a great choice.

“He understands the world for the dangerous place it is,” Graham said Wednesday on Capitol Hill. “He’s got great negotiating skills as our hostage negotiator, and I think he’ll be a very sound policy adviser to the president.”

O’Brien served in other roles at the State Department, in both the George W. Bush and Obama administrations. A former major in the U.S. Army Reserve and a lawyer, O’Brien helped train judges and lawyers in Afghanistan's criminal justice system. O’Brien served as a U.S. representative to the U.N. General Assembly in 2005, working with Bolton, who was the U.S. ambassador to the United Nations.

O'Brien has also worked on several GOP presidential campaigns, including Mitt Romney's failed 2012 bid against Barack Obama.

Speaking with reporters last week, Trump said he spoke with Pompeo about taking on the national security job as well, but the secretary of State declined the idea.

"We have a lot of the same views, and a couple of a little different views," Trump said of Pompeo. "But he likes the idea of having somebody in there with him, and I do too."