Development Gag: Lucas writes a book about his time growing up in Tree Hill, called "An Unkindness of Ravens", the original title for the series. In Season 6, the book is adapted into a film which is ultimately cancelled, with Julian instead adapting the book into a television show in Season 9, just like One Tree Hill started as a movie before Mark Schwahn adapted his pitch to be a television series.
I Am Not Spock: In one interview, Paul Johansson tells the story of one fan who hit him with her handbag because he was "so mean to that boy" and he had to explain to her that he was an actor and not Dan Scott.
Mean Character, Nice Actor: Try to like Dan Scott. Go on, try. Watch some behind-the-scenes features, however, especially gag reel moments with Jackson Brundage (Jamie), and you'll see Paul Johansson is a perfectly likable guy.
Romance on the Set: Chad Michael Murray (Lucas) and Sophia Bush (Brooke) were married in real life, but divorced (after which came one of the seasons where Brucas was in full swing; talk about awkward...). Bush was afteward linked to James Lafferty (Nathan). After she split with Lafferty, she moved on to Austin Nichols (Julian). There's also the reason Murray and Bush broke up in the first place—Murray flirting with and, after the divorce, starting a relationship with a much younger extra who was one of the cheerleaders...
Logan's Green Lantern ring. Doubles a Stealth Pun, as he gives it to Clay & Quinn as an engagement ring after he gets over his fear of the ocean. The ring specifically, is a piece of tie-in merchandise for the 2011 film.
A long standing rumour amongst the Brucas fans is that Brooke & Lucas were meant to stay together, but Chad Michael Murray & Sophia Bush's real-life divorce led to the producers ressurecting the Lucas/Peyton ship instead. There has never been any word or evidence from anyone involved with One Tree Hill to confrm this, with Word of God repeatedly asserting it was always Lucas & Peyton.
After the airing of episode 9x07, featuring the return of Chad Michael Murray as Lucas Scott, Sophia Bush was forced to come out & dismiss rumours that she had asked not to have any scenes with Murray, noting that she actually said told Mark Schwahn to write scenes with Lucas meeting with the rest of the old gang if he wanted.
One Tree Hill existed for several years as a pitch for a film titled "An Unkindness of Ravens".
The series was originally going to be set in Illinois, the home state of creator Mark Schwahn. Executives suggested the producers look at warmer states, and the production settled in North Carolina, which allowed for the show to use Wilmington (The town where Michael Jordan grew up & base for Capeside, Massachusetts) as the base for Tree Hill.
Haley (Bethany Joy Lenz/Galeotti) was not in the original pilot. Instead, it had a different character named Reagan portrayed by a different actress.
Chad Michael Murray was originally slated to play Nathan, which would explain why Lucas is blonde like Deb instead of a brunette like Dan and Karen.
Quinn was originally only going to be around for a year.
Peyton & Lucas were intended to return for Brooke's wedding in episode 8x13, but those plans were dropped when scheduling conflicts meant that neither character was able to return.