This is probably one of the most authentic Cantonese restaurants in Arlington. However, even though the dishes are all authentic, from their "barbecue" meats, to their noodle soups, right down to their casseroles, I came away slightly underwhelmed. Fundamentally, the problem with X.O. is one of execution: dishes came to us either under-seasoned, over-salted, or over-cooked.
The mixed barbecue platter of roast duck, BBQ pork, and soy sauce chicken was merely ok. While the BBQ pork was nicely prepared with a subtly sweet marinade, the soy sauce chicken was pretty ordinary while the roast duck was tasteless. Our steamed flounder with ginger-scallion sauce was overcooked, dry, chewy, and bland. Similarly, the seafood and eggplant casserole did not have a lot of flavor. On the flip side, the congee with preserved egg was way over-salted. It probably isn't a great sign when the best dish is comprised of stir-fried snow pea leaves with garlic (which were very good).
I heard that this place was owned by the same people as Full Kee in Chinatown, and the menus are very similar (X.O. eschews many of the Americanized dishes found in Full Kee). The eggplant casserole, for example, was almost identical to the casserole I got once at Full Kee (they even use the same casserole dishes), except the one at Full Kee had more flavor. It feels odd to recommend a DC Asian restaurant over a NoVa restaurant, but to be honest, the kitchen at Full Kee is probably a little bit more skilled than the one at X.O., and the dishes are pretty much the same. So, you might as well get the better prepared dishes at Full Kee and save yourself the trip to the burbs. I think this is a testament to the sad state of affairs of Cantonese cuisine in the greater Washington metropolitan area.