oc seafood brunch
I'm assuming that you mean a buffet brunch, not a sit-down brunch. I'm not aware of any such brunches that are specifically seafood brunches.
Orange Hill Brunch is very good, and has a lot of seafood on it, although it is not exclusively a seafood brunch. In addition, last time I checked, there was a lot of seafood on the brunch at the restaurant at Le Meridien hotel in Newport Beach. About 5 years ago, that was the best brunch in OC -- now it's called Sutton Place Hotel, and I understand that the brunch is about as good as it used to be (though I think Orange Hill has a better brunch). As an "every day"-type Sunday brunch, I think that El Torito Grill in Fashion Island has a good one. However, the seafood on that brunch may be limited to fresh shrimp, ceviche of two kinds, and maybe two or three hot seafood entrees.
Duke's at the Huntington Beach pier has a brunch with lots of seafood items. IMHO their food is average, but they do have a selection of Hawaiian fish on their regular menu. I think the Hilton Hotel near there also had (used to have?) a Sunday brunch emphasizing seafood.
I've been to the buffet at the Hilton. The restaurant is Cielo Mare. The buffet was OK, but nothing that made me want to go back. Nothing spectacular on the seafood end of things, either. I've never been to Duke's (for brunch or otherwise), but it seems to be very crowded whenever I am there and want to try it.