I don,t grow any olive trees, but they can withstand temperatures down to -5deg c without any winter protection, any protection could probably bring this down a few degrees, but i doubt they could withstand -15 without any problems. best of luck

I agree - you'd get away with it probably in a sheltered, preferably walled area, south facing, out of cold northerly and easterly winds, but its a risk nonetheless. We have them here in the South, but generally grown in pots, which can then be moved to more sheltered spots for cold winters.