What's a good replacement for oyster sauce in asian recipe..?

I don't know of any good replacement since most stir-fry recipes (at least the ones I use) call for soy sauce, oyster sauce, and sesame oil. So, you could just leave it out. You might try just a little Worcestershire sauce in place of the oyster sauce. Teriyaki and Hoisin are sweet. The first two ingredients on Oyster Sauce is water then sugar. So, you may be able to try either of those if you want the sweetness.