In episode 7x08, the guys discuss their favorite sports movies. Skills, played by Antwon Tanner, mentions Coach Carter as his—Antwon Tanner was one of the stars, and Mark Schwahn, the show's creator, was a co-writer. Nathan, played by James Lafferty, mentions Hoosiers, and one of Lafferty's early roles was in a movie based on the Hoosiers.
One of Peyton Sawyer's defining characteristics is her love for music - Hilarie Burton was working for MTV as a VJ prior to being cast as Peyton, and continued in the role alongside her shooting schedule for One Tree Hill for a time.
During Lucas & Peyton's wedding reception in episode 6x23, Julian rants about Brooke bringing Nick Lachey as her date, asking how he can compete with "the guy from Dancing with the Stars" & when Missy points out that he's thinking of Nick's brother, Julian then snaps "Laguna Beach then!" which Chase retorts was "Some other tool." The tool in question was Stephen Colletti, who plays Chase.
The Danza: Among the bit players, it's happened a couple times. Skills Taylor, whose actual name is Antwon, is played by Antwon Tanner. Bevin Mirskey, his one-time girlfriend, is played by Bevin Prince.
Dawson Casting: The show itself was an example of this (the only one close to high school age was James Lafferty, who was 18 when production started) during the first four seasons. Part of the reason for the four-year jump was so the actors could play characters close to their ages. However, the practice was amusingly lampshaded during Season 6, when Lucas' book about their high school years was becoming a movie. Everyone joked about who they wanted cast in their role, and when Lucas replied they were supposed to be high school students, the response was usually "But have you seen how old the actors playing high school students are these days?"
Zig-zagged with Stephen Colletti (Chase Adams) - Like the rest of the cast, he was an example of this when Chase was introduced in Season 4, being 20 years whilst Chase was a high school senior. After the 4 year Time Skip, he was an inversion, as he was now 2 years younger than the character.
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.
Dyeing for Your Art: God knows how many hairstyle/color changes there have been between seasons. Just try to keep track of Haley's hair color changes and styles (dark brown, light brown, blond-ish, long, short...). Brooke goes from no bangs in Season 1 and 2 to bangs in Season 3; Peyton's curls and shade of blond varied between seasons. The boys had their fair share of changes, too.
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.
Irony as She Is Cast: Lucas Scott is one of the best basketball players in the Ravens line-up... But Chad Michael Murraycannot play basketball. In the DVD Commentary for the first season finale, the cast all get a good laugh about how the one time Chad was supposed to miss, he couldn't stop putting it in the hoop; and the Season 5 Gag Reel features a montage of Chad's misses, hilariously matched up with 6-year old Jackson Brundage repeatedly making free-throws.
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.
The Other Darrin: Peyton's dad Larry switches actors between Season 2 and Season 3. Guess Peyton doesn't notice he looks a little different when he comes back from sea...
Whilst it has all the hallmarks of a Postscript Season, this wasn't the case with Season 5 - Season 4 wraps up most of the loose ends from the first 3 seasons, and ends with the main characters graduating from high school whilst Dan Scott begins his prison sentance for murdering his brother Keith, and Season 5 saw a massive retool thanks to the 4 year Time Skip; Mark Schwahn states on the Season 5 DVD that when they were planning Season 5, it was decided to go with the Time Skip because they'd already done most of the typical college storylines whilst the characters were in high school, and it allowed more freedom with the plot since they could pick up with the characters returning to Tree Hill after college & they would be able to jump right into Jamie starting school rather than having him age naturally.
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...
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.