British OO Scale: (1:74) Primarily used by British railroad modelers. Most manufacturers of OO scale will fudge a little on the gauge so that the locomotives and rolling stock will run on HO track. The British also use 4mm finescale standards of EM gauge (18.2mm) and P4 (18.83mm), which is an attempt to be more prototypical and less toy-like, but requires more skill to model. Check out the British Railway Modellers of North America for more information.
American OO Scale: A model railroad standard gauge with a scale of 4mm=1 foot (1:76), slightly larger than HO scale. It utilizes 19mm (0.748 in) for the standard gauge track. It will not run on HO track like the British OO will. It will usually run on On30 track, since the difference is gauge is minimal.