CLEVELAND — It all started with a tie. Or, to be more precise, a whole bunch of ties.

The rapport between Donald Trump and Oklahoma oil billionaire Harold Hamm, who addressed the Republican National Convention on Wednesday, has an unlikely sartorial beginning.

The two men had crossed paths over the years but first got to know each other in late 2012, Hamm recalled in an interview here Monday. The chief executive of Continental Resources was in New York on business and, at Trump’s urging, dropped by Trump Tower to say hello. Over the course of half an hour, the real estate developer quizzed Hamm on oil-drilling techniques and energy policy, taking notes as he spoke. Trump even floated the idea that he might run for higher office. Then he observed that Hamm was not wearing a tie.

“You know about Trump ties?” Trump asked, as Hamm recalled. “Did you know they are a best-seller? You ought to have a sample or two.”

Trump took Hamm down to the gift shop in the lobby of Trump Tower, where a rainbow of ties was arrayed in a display. The developer started pulling out ties and handing them to Hamm. “You like red?” he asked. “You like blue? You like yellow?”

Hamm went back to Oklahoma City with a collection of Trump ties. And in early 2014, when he was set to be photographed for a Forbes magazine cover story, he happened to put one on.

A few days later, a package arrived from Trump. Inside were more ties and a note on his personal stationery:

“Dear Harold: Your tie looked great on the cover of Forbes, and the story was even better. You are amazing!

With Best Wishes,

Donald J. Trump

P.S. I am enclosing some more ties for you.”

The gesture impressed Hamm.

“He is extremely thoughtful and he has great insight,” he told The Washington Post Monday. “I’m not blown away by people. I’m really not. I’ve been around very, very important people throughout my career. I’m not blown away by celebrity at all. But you get to where you can recognize good, genuine people. He’s certainly one of those. He’s original – I like that.”

This April, Hamm endorsed the real estate tycoon’s presidential bid and called him the “business leader’s candidate.” He now serves as one of Trump’s unofficial energy advisers, appearing with him at a policy address in North Dakota, where Trump dubbed him “the king of energy.”

Hamm, who was an energy adviser to 2012 GOP presidential nominee Mitt Romney, said he actually speaks with Trump more frequently than he did with Romney.

“What we talked about, a lot of that has come true over the past four years,” Hamm said. “And so I think he has a great deal of confidence in what I tell him.”

He said he was surprised when Trump asked him to speak at the convention, but “if I could help, I wanted to,” Hamm said. He plans to talk a bit about his personal story: the youngest of 13 children of Oklahoma share-croppers, he kept out of school as a youth to pick cotton until the first snow covered the fields. He launched his own company at age 20, and has since grown it into one of the country’s largest oil producers.

Continental Resources chief executive Harold Hamm and former United States Secretary of Labor Elaine Chao will discuss the GOP'S new platform on trade -- a key issue in Ohio, the US and abroad. (TWP)

“I think it’s very important to talk about the American dream and the restoration of that dream for American youth,” Hamm said.

Even after a drop in global oil prices, the Continental Resources chief executive is still worth an estimated $11.9 billion, according to Forbes. He has the capacity to put huge sums into a super PAC supporting Trump, though Hamm said he is focused on writing more limited checks to the Republican National Committee.

“Oil is $46 a barrel, so our industry is hurting for a lot of reasons,” he said. “It’s not a year when we can be overly generous. But I am going to be supportive.”