... on the crystals sometime around 1950. Some of the very early bumper movement watches might still have original crystals without a logo. Also, I'm not sure if the US-cased gold-filled and 14K models would have originally come with logo crystals, since those crystals may not have been made by Omega.
However, Hoi is correct that any modern acrylic crystal from Omega is going to have the logo in the middle. By this time, most vintage watches will have had the crystal replaced at least once.
Hope this helps,
gatorcpa