From what I can see, Camarillo (CMA) (12.6 mi) or Oxnard (OXR) (19.0 mi) are the closest, with Van Nuys (VNY) (22.1 mi), Whiteman (WHP) (26.9 mi) and Bob Hope (Burbank) (BUR) (29.6 mi) next. If you want an airport with major airlines, then it is Los Angeles Intl. (LAX) (32.3 mi). Those are straight line miles, not road miles, so expect some differences. Those are all based on the lat/long of the center of Thousand Oaks to the center of the respective airports.