Shinobu Shabu on Goldenwest is really good. Au Lac in Fountain Valley is worth checking out (vegan). Plum’s in Costa Mesa is also really good, but probably not a “hidden gem”. Coconut Rabbit (Thai) in Cypress can be good, too.
Though it’s not officially in OC, Zuzu’s Petals in Long Beach rocks my world.
Edit: Amarinth Table in Garden Grove is wonderful.