Besides Oasis (no 1st-hand experience with them), full-service bike shops I've used and had good experiences with: Spokes, Etc. (Vienna), Bikes@Vienna, A-1 (Herndon). Spokes has locations in Alexandria that may be more convenient, and A-1 has a location in Manassas (no experience with them). I have heard good things about Bike Lane (Burke, Reston Town Center) but have only used the RTC store for odds n ends.