O gauge 1/48 was originally 0 (zero or zed) gauge. Number 1 gauge was 1/32 and number 2 was 1/24.
Because of the smaller sizes or European and English trains they went and turned O into 1/45 and 1/43,5 respectively in order to fit motors then available. When the English created HO it became 1/87, but then they discovered the trouble of fitting motors in their locomotives and created OO 1/76, using either 16.5 mm HO track, or the correct 18 mm OO track.
Stuart