Mischa Barton Claims She Dated Ben McKenzie While on 'The O.C.'

Mischa Barton claims she dated her one-time 'O.C.' costar Ben McKenzie -- and it's troubling since she says she was underage at the time ... a window she calls "complicated."

The actress shared her story on "Call Her Daddy" this week, saying that the onscreen relationship she shared with Ben actually took hold in real life as well -- something she alleges happened right from the jump when she first joined the show.

Check it out ... MB says she was 17 at the time, and that Ben was a few years older in his mid-20s -- and notes she was very new to all of it, as she hadn't dated much before him.

Safe to say ... the age difference came onto the radar of producers who were apparently concerned about it -- at least according to Mischa, anyway. She says showrunners were hitting up her parents ... and it was a "whole ordeal," as she puts it.

Mischa goes on to say she and Ben only dated for a bit before breaking things off -- and it sounds like she might've dated other costars as well ... explaining that a lot of people on "The O.C." back then were hooking up with each other.

Long story short, Mischa says she had to grow up fast while working on the series -- although, it doesn't sound like all of it was exactly smooth. The 2000s were a different time, indeed.