Brian O’Neill hadn’t yet started a game for the Vikings when he began introducing himself to established NFL stars.

The whistle had blown on a play during the Vikings’ 23-21 victory in Philadelphia on Oct. 7 when the rookie tackle put his hands on the chest of Eagles defensive standout Fletcher Cox.

The seconds that followed might be the best example of why O’Neill was labeled “fearless” by one team captain.

With the quickness of a three-time All-Pro, Cox whisked O’Neill’s hands away. He responded with a step forward, sharp wit and a brush of the facemasks.

After the game O’Neill’s high school basketball coach, Brendan Haley, texted him: “I saw you jawing. It hasn’t stopped since high school.”

“He was like, ‘Yeah, [Cox] couldn’t say anything back!’ ” Haley said. “I said look, this is the pros. It’s Fletcher Cox, keep it under control. But he just loves that stuff. I’m sure that’s why his teammates love him.”

So this week, O’Neill is leaving the fawning over his primary opponent, Bears edge rusher Khalil Mack, to fans and media.

The 23-year-old prepares to make his fifth NFL start on Sunday night in Chicago with a mental edge fit for an offensive lineman, and one he’ll need against Mack. The Bears move around Mack, the overpowering outside linebacker, but he rushes quarterbacks most where O’Neill will be at right tackle (64.8 percent), according to Pro Football Focus.

“I don’t really get caught up in who’s who or what somebody is supposed to be or not,” O’Neill said. “I just try to go out and do my job the best I can.”

O’Neill is the well-spoken nephew of Delaware Gov. John Carney. But his sharp tongue was the first muscle he developed as the youngest of four siblings to Brendan and Liz O’Neill. He was the rare sophomore to start in varsity football at Salesianum, an all-boys Catholic school in Wilmington, where O’Neill’s teams won Delaware state titles in football and basketball.

Stories about O’Neill’s trash talk abound from his high school coaches, Haley and Bill DiNardo (football), but it’s not rooted in arrogance. Charisma lifted O’Neill into being the leader on a state title-winning basketball team also featuring current Milwaukee Bucks guard Donte DiVincenzo. He knew when to confront or build up teammates, according to Haley, and when to defend them or himself against opponents.

“He talks more stuff than any human being I know,” DiNardo said.

That on-field confidence has eased his NFL transition. This season is only O’Neill’s fourth playing offensive line.

The former high school receiver and college tight end moved to tackle for the University of Pittsburgh in 2015. O’Neill is still adding weight to his 6-7, 297-pound frame, which should help him as a run blocker in seasons to come. His nimble feet and hands — basketball-tested — have already flashed.

Even as a second-round pick (62nd overall), O’Neill’s trajectory into the lineup was hard to predict when he struggled in August training camp, bouncing between the Vikings’ No. 3 and No. 4 tackle on the preseason depth chart.

He’s since turned his first NFL start, replacement duty in Week 6 vs. Arizona, into the full-time job at right tackle. Only five other NFL tackles have played as much as O’Neill (382 snaps) and not yet surrendered a sack this season, according to PFF. However, his 16 quarterback hits and hurries allowed are the most among them.

“Everybody says the game slows down, but really it’s your process speed picking up,” O’Neill said.

In-game adjustments have been the biggest improvement for O’Neill, according to defensive end Stephen Weatherly, who described O’Neill catching on to Weatherly’s “long-arm” technique and developing an effective countermove in practices.

“We’ll get into these long battles,” Weatherly said. “Just colliding like two rams.”

However, many Vikings defenders say they haven’t heard O’Neill jaw with teammates at practice. It sounds like the rookie knows when he should — and shouldn’t — pull out his verbal countermoves, too.

“That part is fun,” O’Neill said. “It’s not going to be a distraction. I’m not going to let it get out of hand, but if somebody is going to say something, I’m not going to walk away. I probably should, but I’ll probably say something back.”