In the United States, O gauge is defined as 1:48 (0.25 inches to the foot, quarter inch scale - 1/4 inch equals one foot). This is also a common dollhouse scale, giving more options for buildings, figures, and accessories. Many O gauge layouts are also accessorized with 1:43.5 scale model cars.

O gauge (or 0 gauge) is a popular model railway standard that historically originated as "Scale 0," being smaller than Gauges 1, 2, and 3. In the 2026 modeling community, the technical size of O gauge track is defined by a 32 mm (1.26 inches) distance between the rails. However, the "scale" (the ratio of the model to the real world) varies by region: in the United States, O scale is typically 1:48, whereas in the United Kingdom and much of Europe, it is 1:43.5 or 1:45. This means that while the tracks are the same width globally, a British O gauge locomotive will actually be slightly larger than its American counterpart. O gauge is famous for its durability and "play value," popularized by brands like Lionel in the mid-20th century. Because the scale is large enough to allow for intricate details—such as working steam smoke, realistic sound systems, and opening doors—it remains a favorite for hobbyists who have the space for a more substantial, tactile layout compared to the much smaller HO or N scales.