O Scale (or O Gauge) is a scale commonly used for model trains and miniature sets. O gauge was the most common model railroad scale in the United States and remained so until the early 1960s. Detail and realism were secondary concerns, at best. It still remains a popular choice for those hobbyists who enjoy running trains more than they enjoy other aspects of modeling

The size of O is larger than OO/HO layouts, and thus is a factor in making the decision to build an O gauge layout.