OO gauge or OO scale (also spelled 00 gauge and 00 scale) model railways are the most popular standard-gauge model railway tracks in the United Kingdom. This track gauge is one of several 4mm-scale standards (4 mm:foot / 304.8 mm or 1:76.2) used, but it is the only one to be served by the major manufacturers. Despite this, the OO track gauge of 16.5 mm (0.650 in) is inaccurate for 4mm scale, and other gauges of the same scale have arisen to better serve the desires of some modellers for greater scale accuracy.
Read more about OO Gauge: Origin, Scaling, OO Today, 4 Mm Finescale Standards