lol... they totally do fit! If I do write this (I'm so tempted since I assume you at least would read and enjoy...lol) obviously the OTH and The OC characters won't be forced directly into the molds of the HG characters, but in general I've currently got:
Ryan - Scott (Julie is totally his evil child-molesting stepmother... lol)
Brooke - Shelby (It's not a big leap from ignoring your child to abusing them...)
Marissa - Juliette (Although poor Jules deserves better... lol)
Seth - Ezra (He does have the right kinda personality...)
Peyton - Daisy (Although I dislike Peyton and LOVED Daisy... but the dark and dreary personality... lol)
Lucas - Auggie (Not a 'bad' kid, just one who makes stupid decisions and gets in trouble...)
Haley - Kat (The kid who's ready to graduate the program, like the 'mom' of the kids...)

I could always create characters for the OTH and The OC charatcers who aren't on the above list... lol... Nathan and Rachel from OTH, Luke, Summer, and Alex from The OC all pop into my mind...

And then, depending on where my mind goes, Sandy and Kirsten could be put in the roles of Peter and Sophie, the teachers at the school who genuinely care about the kids and wind up falling in love with eachother... lol...