Scaling
16.5 mm (0.650 in) gauge at 4 mm:1-foot means that the scale gauge represents 4 ft 1 1⁄2 in (1,257 mm), 7 inches (178 mm) narrower than the prototype 4 ft 8 1⁄2 in (1,435 mm). This noticeable difference are aggravated if over-scale rail section, over-scale wheel width and deep wheel flanges are used on typical models. These departures from scale require much larger clearances on pointwork and are particularly noticeable when looking along the track. This scale gauge more accurately represents the narrow gauge railways built to 4 ft (1,219 mm) gauge, for example the Padarn Railway and Saundersfoot Railway in Wales and the Glasgow Subway in Scotland.
Though they run on the same track, OO gauge and HO gauge models of the same prototype do not sit well together since the OO models are larger than the HO equivalent.
OO is also used to represent the 5 ft 3 in (1,600 mm) Irish gauge, where it is a scale 131⁄2 inches (343 mm) too narrow.
These differences have led to the development of the finescale standards EM gauge and P4 scale.
Read more about this topic: OO Gauge