O scale (or O gauge) is a scale commonly used for toy trains and model railroading. Originally introduced by German toy manufacturer  Marklin around 1900, by the 1930s three raill alternating current O gauge was the most common model railroad scale in the United States and remained so until the early 1960s. In Europe, its popularity declined before World War II due to the introduction of smaller scales.

O 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 secondary concerns, at best. It remains a popular choice for hobbyists who enjoy running trains more than they enjoy other aspects of modelling, and collecting vintage O gauge trains is also popular – there is a market for reproduction and vintage style models also. A number of changes in recent years have addressed the concerns of scale model railroaders, making O scale more popular.