On30 uses the American O scale of 1⁄4 inch to the foot, (ratio 1:48) to operate trains on HO gauge (16.5 mm / 0.65 in) track.

In the United States modelling in On30 dates back to the 1950s, using HO gauge wheels and locomotive chassis.

[1] An On30 layout, the Venago Valley (built by Bill Livingston) was featured in the June 1971 issue of Railroad Model Craftsman magazine.

In 1998 Bachmann Industries introduced a model of a 2-6-0 steam locomotive in this scale for the Christmas village market.

On30 is now regarded as the fastest growing segment of the model railroading market in the United States.

The second group model prototype American narrow gauge railroads ranging from mining and logging companies through to large shortline railways such as the 2 ft (610 mm) Sandy River and Rangeley Lakes and the 3 ft (914 mm) Denver and Rio Grande Western Railroad.

For accurate modelling of two-foot gauge railways, such as the Ffestiniog or Lynton and Barnstaple O14 has been employed, although there is some limited commercial support for this scale/gauge, it is mostly of industrial prototypes.

A similar pattern of small manufacturers producing kits is followed in Continental Europe.

[5][6] Recent examples include 'Steam in the Bush' who are based in the Blue Mountains and produce a range of On30 'craftsman' style kits.

A small number of models have also been produced in 7 mm:1 ft scale, mostly of New South Wales prototypes.

The following table lists the most popular narrow gauges in O scale: 5 Model railroaders with layouts and rolling stocks by American standard and usually by British standard, use for designation of the scale in English language publications the letter O and not the number 0.