Tonight's show featured Seth Cohen's holiday "Chrismukkah". This combined holiday made its appearance last year during the shows first season and it didn't take long for it to catch on. Only a year later, Chrismukkah has made it's way into pop culture and everyday conversation... it has even been commercialized - Chrismukkah items are now for sale (to purchase your own items, check out The OC Insider). The above link is to a USA Today article which describes the trend and merchandise in greater detail.
For more OC info and to read recaps of last season's episodes to refresh your Chrismukkah vocab and memory, check out the show's official site.