There is a place in San Clement called Tommy's that comes closer than anything else I have ever seen. It has both the typical hugely varied menu and the all importantt Greek specialities. The chicken souvlaki is particularly good. The decor is a little odd - an over the top Holywood theme. However, there is nothing self conscious or dineresque about the food. Good burgers, homemade soups, chili, breakfasts.
It has a very high sign clearly visible form the 5 - drove by it for years without stopping. Soemone recommended ti to my husband and we are converts.
But your best bet for a casual place serving incredibly delicious (almost etherally so) food in the OC is to go immediately to Break of Dawn Cafe in Laguna Hills. Only open for breakfast and lunch.
Break of Dawn
24351 Avenida De La Carlota Ste N6, Laguna Hills, CA 92653