"I got to go to work on a really big-time show, so I was thrilled every moment. It was definitely a whirlwind learning experience. I left high school and kind of dove straight into it. It was a lot all at once — but very positive," she told news.com.au ahead of her visit to Australia for this weekend's Oz Comic-Con Melbourne.
With a cast made up of hot young stars including Ben McKenzie (Ryan), Adam Brody (Seth), Rachel Bilson (Summer) and Mischa Barton (Marissa), the show's entire four-year run was riddled with rumours of on-set romances and rivalries.
While Holland, now 27, was cagey about naming exactly who was hooking up with who — although Brody and Bilson's relationship is well-documented — she confirmed it was definitely happening.
"Some people were dating, some people loved each other, some people had certain relationships where they didn't really like each other … When I got there, it was already a quite well-developed cast, everyone had kind of gone through all the motions together," she said.
Without a doubt, her fondest memories are with castmate Peter Gallagher who played everyone's favourite TV dad Sandy Cohen — and who allowed plenty of that on-screen personality to spill over into real life.
"He was just so welcoming, with open arms. He was just the father of everyone, it was really nice to be introduced by him to the whole thing," Holland explained.
"Everyone offered me little bits of advice all the way, but everyone's schedule was so busy — it was like moving parts constantly."
In season 3, Holland's character returned to the show (although she was the second actress to play Kaitlin, with a much-younger Shailene Woodley cast as the original in season 1) to essentially cause chaos for her older sister and her group of friends.
She introduced Seth to marijuana and essentially blew up Marissa and Ryan's relationship by hanging out with Marissa's friend Jonny who everyone knew was madly in love with the older Cooper.
It all culminated in a drunken night on a beach clifftop, where Jonny and Kaitlin hit the bottle and Jonny ends up falling to his death (teen dramas, eh?).
While plenty of fans never forgave Kaitlin for her role in the brutal demise of the TV hottie, Holland admitted there was one incident off the main street in New Orleans that went way too far.
"I got jumped by a girl who was near blackout drunk — she ran and jumped on my back and pushed me to the ground and then was on top of me," the actress told news.com.au.
"She wasn't trying to hurt me, she was trying to shock and scare me and was yelling in my face, 'You killed Jonny, you killed Jonny!'. It was really intense.
"I had to have a bunch of people pull her off me."
It was certainly an interesting time for Holland to have joined the show, given the drama surrounding co-star Barton's exit.
The young actress was quickly and easily the biggest celebrity on the show. After just two-and-a-half seasons, Barton clearly felt she was destined for bigger and better things and reportedly pressured the writers to kill off Marissa.
"I think I just got to the point where I was like, 'I'm not sure I'm enjoying this anymore'," Barton told the US ABC network in 2016.
"Everybody was depending on me … I asked to get out of (promotional) jobs all the time, and the response was, 'No, you have to'. There's an attitude that you can't say no."
Despite reports Barton's departure was acrimonious, Holland was reluctant to weigh in on her former co-star.
"That's quite personal to her — it's not my place to comment on that," she told news.com.au.
Besides, at just 14, Holland was well and truly the youngest cast member when she joined in 2005 and didn't spend a lot of time with her co-stars.
Barton was the closest to her in age at 20, McKenzie was 27, Brody was 26 and Bilson was 24.
It meant that while there was plenty of partying and romance happening around her — she wasn't exactly allowed to take part.
"I was so young and there was such a large age difference between all of us — Ben McKenzie was almost 30," Holland said.
"So, for me, there wasn't a lot of hanging out off the set because I had to go home and do my homework and stuff and be a kid."
She may have only appeared on The OC for two seasons, but the show certainly kickstarted Holland's career.
A year later, she landed a recurring role in another cult teen drama — Gossip Girl. Holland played Taylor Momsen's wild-child model sidekick Agnes Andrews, another troublemaker not all that different from Kaitlin Cooper.
Her biggest role in recent years has been as Thea Queen in Arrow and The Flash.
