O gauge is a scale commonly used for model trains and railroading. Originally introduced by German model train manufacturer Marklin around 1900, by the 1930s three-rail alternating current o gauge was the most common model railroad scale in the USA and remained so until the early 1960s.
In Europe, its popularity declined before WWII due to the introduction of smaller scales. Britain stuck to a lower voltage direct current.
0 gauge had its heyday when model railroads were considered toys, with more emphasis placed on cost, durability, and the ability to be easily handled and operated by pre-adult hands. Detail and realism were not so important.
O gauge remains a popular choice for hobbyists who enjoy running trains more than they enjoy other aspects of modeling, and collecting vintage o gauge trains is also popular. In addition, a number of changes in recent years have addressed the concerns of scale model railroaders, making o scale more popular, at least in the United States.
In the United Kingdom it is more popular amongst finescale modellers who prefer to make perfect models than run trains- 00, because of its low price and high availability is the choice for those who enjoy running their trains more.
Find out more at Wikipedia.