There are so many reasons to love The O.C.'s Seth Cohen (Adam Brody): his endearing dork-outs. His rockin' taste in music. And Chrismukkah, the best made-up holiday since Festivus. Like all good holidays, Seth's Christmas-Hanukkah hybrid (his dad's Jewish and his mom's a ''WASPy McWASP'') is already getting commercialized. Just in time for the Fox series' Dec. 16 Chrismukkah episode, www.theocinsider .com is offering seasonal trimmings, including greeting cards, wrapping paper, and faux-fur-trimmed yarmuclauses. ''Chrismukkah has become part of our cultural zeitgeist,'' says Warner Bros.' Lisa Gregorian of the fast-selling goodies. ''That's what makes The O.C. so genius.''