(CNN) President Donald Trump once said his then-chief hostage negotiator and now newly installed national security adviser Robert O'Brien praised him as being the "greatest hostage negotiator that I know of in the history of the United States."

But despite O'Brien's more recent flattery, a CNN KFile review of O'Brien's past statements in public, radio and Fox News appearances indicates that he was once highly critical of Trump when he was running for president in 2016.

O'Brien, a lawyer and former diplomat in the George W. Bush administration, eventually supported Trump in the fall of 2016. But before he did, O'Brien criticized everything from Trump's perceived chumminess with Russian President Vladimir Putin to his comments about Carly Fiorina's appearance.

Notably, in March 2016, O'Brien called Trump's foreign policy statements about US allies, including those in NATO, "extraordinarily dangerous."

"NATO (has) kept the peace for two generations since the war. And Donald Trump is, has said that he's willing to scrap our alliance with NATO. He's willing to scrap our ... alliances in the Pacific with Japan and Korea and South Korea. I mean, these are extraordinarily dangerous statements at a time that the world does not need that sort of inconsistent and wavering American leadership given the threats that we're facing," O'Brien told Fox News , adding that Trump's "policy prescriptions are not Republican or not traditional Republican policy prescriptions."

O'Brien is far from the first former Trump critic to join the President's inner ranks, which now include an energy secretary who once called him a "cancer on conservatism," a key ally on Capitol Hill who once said he was a "race-baiting, xenophobic bigot" and an acting chief of staff who labeled him a "terrible human being."

Asked for comment, O'Brien told CNN, "Despite attempts to misconstrue political analysis during a presidential election as evidence that I'm somehow opposed to the President -- nothing could be further from the truth."

"I'm honored to have voted for President Trump and serve in his Administration, and to proudly work to carry out his foreign policy agenda that protects all Americans, our allies across the globe, and puts America first," he added.

Criticized Trump's foreign policy statements

O'Brien had criticized Trump's foreign policy positions several times before lending his support to the eventual Republican nominee.

While serving as a presidential campaign adviser to former Wisconsin Gov. Scott Walker and later Texas Sen. Ted Cruz, O'Brien was concerned with Trump's rhetoric around foreign policy -- especially NATO and Russia.

In an October 2015 Politico op-ed , O'Brien advised Republican primary candidates to counter Trump's foreign policy views, specifically challenging Trump's perceived chumminess with Putin.

"(Cruz) certainly has an opening to do so on national security versus Trump, who has been playing up how chummy he will be with Vladimir Putin if he is elected," O'Brien wrote.

O'Brien said in February 2016 that Trump's statements about defense budget cuts were "very troubling."

(...) he has not issued a plan or a program to rebuild America's military other than the platitudes. I hope there's something," O'Brien "Well, no one likes waste, fraud and abuse, but there's not enough waste, fraud and abuse to find in there to make sure that the guided missile cruisers get modernized on time and get back into the fleet. So, you know, I hope he means what he says, but so farhe has not issued a plan or a program to rebuild America's military other than the platitudes. I hope there's something," O'Brien told the Hugh Hewitt Show

In April 2016, O'Brien suggested on Fox News' "Strategy Room" that Trump was in a " bromance " with Putin.

In the same appearance, he reiterated that he thought there was "some real discomfort" within the GOP about Trump's rhetoric around NATO and Putin.

Dismissing Trump's potential presidency

O'Brien said on Fox News in April 2016 that Trump lacked a president's temperament, saying his new nickname for Ted Cruz at the time -- "Lyin' Ted" -- didn't "look particularly presidential."

O'Brien also expressed concern Trump "could lose by double digits, especially given the standard among women and including Republican women."

During one appearance on a panel in the spring of 2016, O'Brien was highly critical of Trump, saying the then-front-runner in the Republican primary could lose all 50 states.

"In the short term, things don't look great for the GOP," O'Brien said at a Pacific Council event . "We have a front-runner who could lose all 50 states if he's the nominee. I do think it will be a contested convention, but that doesn't mean that Trump can't win."

The former State Department official also emphasized Trump's previous allegiance to the Democratic Party, telling Fox News that Trump wasn't a Republican "until he claims he converted a couple of years ago."

In that same March 2016 appearance, O'Brien was skeptical that Trump would reach the delegate support needed for him to clinch the GOP presidential nomination.

"I think Trump realizes now that he's probably not going to get the 1,237 delegates and the rules of the rules. And if you don't have 1,237, you're not the nominee. If he loses in Wisconsin, which is likely next week, he's not going to have the delegates. And I can't imagine a convention made up of Republican party activists and people that are committed conservatives on a second, third, or fourth ballot turning to Donald Trump," O'Brien said at the time.

But by July, O'Brien appeared to admit that Trump's lack of a presidential temperament would not cost him the nomination.

"(W)hile (Republicans) may have been concerned about either his temperament or some of the policy statements, they're going to turn to Trump as he's the nominee in two weeks in Cleveland," O'Brien said

Railing against Trump's rhetoric

O'Brien's critiques of Trump expanded far past politics and foreign policy. He was also vocal about Trump's birther claims and comments about women.

He called Trump's birther claims "spurious" when asked about a claim that Cruz wasn't eligible to run for president because he was born in Canada, and Trump's tweet stating Democrats would sue if Cruz was the nominee.

"I think after defending Barack Obama for eight years on birther claims that were just as spurious as the claim being made against Ted Cruz, uh, the Democrats would not be helping themselves politically. And I, I don't think any Hillary campaign manager would want that suit to go forward. It would look, it would look poor for her," O'Brien said in early 2016

O'Brien also called Trump's comments about a judge's Hispanic heritage "ill-advised." In the summer of 2016, Trump said the judge, who was presiding over a Trump University case, could not be impartial because he was "Mexican." The judge was born in Indiana

"It was certainly a mistake. It was an ill-advised comment, and I'll tell you as a lawyer, I would have been just beside myself if one of my clients had made a comment like that about the judge presiding in his case," O'Brien remarked

The new national security adviser also said Trump objectified women, comparing him to former GOP congressman Todd Akin, who dropped in the polls after he claimed that victims of "legitimate rape" rarely get pregnant.

Asked if Trump had objectified two women, including then-Fox News anchor Megyn Kelly and then-presidential candidate Carly Fiorina, during the campaign, O'Brien responded , "He's been doing some of that through the whole campaign."

"They're, they're accomplished women. And I think he's going to have a real tough time in the general (election). We saw this in a Senate race a couple of years back when Todd Akin, I had a difficult, uh, time attracting women voters and lost about 15% of the Republican vote and lost his Senate campaign to Claire McCaskill in Missouri," he continued.