So it was only fitting on Thursday at the NFL Scouting Combine when Hackenberg and O'Brien were speaking at podiums across the room from each other at the same time.
Coincidence? Yes, surely, but no team can match the Texans when it comes to background info on the enigmatic QB prospect.
O'Brien coached the QB during his freshman season at Penn State, before O'Brien left to take the Texans' job. It was the peak of Hackenberg's college career, as his production fell over the next two seasons under O'Brien's replacement, James Franklin. Hackenberg once appeared to be on a track to be a top pick, but now seems slated for the middle rounds. NFL Media draft analyst Lance Zierlein rates him the No. 5 QB in the draft.
On Thursday, O'Brien wasn't interested in offering a draft projection for Hackenberg, but he did give a full-throated endorsement of his character.
"First of all, he's a great kid," O'Brien said. "He comes from a great family. We enjoyed recruiting him. I've known him since he was 17 years old, when he was at Fork Union (Military Academy). I've known him for a long time. I know his parents, his brothers. He's a humble kid. He's talented, smart. Good guy to be around and I enjoyed coaching him his first year at Penn State."
As for how that familiarity plays into the way he views Hackenberg as a prospect, O'Brien dismissed the notion that he'd look at Hackenberg differently from any other QB in this year's class.
"Naturally, I know him a little better," O'Brien said. "It doesn't mean I evaluate him any differently. It just means that I've been around him a little bit more."
On the other side of the room, Hackenberg played the questions about his ties to O'Brien similarly, acknowledging that reuniting with him would be great while also not taking anything for granted.
"I think it's a great opportunity, but at the end of the day, I'm just here trying to be the best prospect that I can be and impress 32 football teams," Hackenberg said.
While O'Brien gave a thumbs-up to Hackenberg's character on Thursday, the most serious questions about Hackenberg as a prospect are related to his skills as a passer. He has the tools, but struggled mightily over the last two seasons at PSU, failing to live up to his billing after O'Brien departed.
If O'Brien determines he can get the most out of Hackenberg at the next level, a reunion could be an awfully hard temptation to resist.