There are two varieties of OO scale model railroad trains - American and British. The American models (referred to as American OO) are 1/76 scale, 3/4 inch (19mm) gauge, sized about half way between HO scale and S scale. The British models are made at a track gauge of 16.5mm, so they're actually HO gauge but with bodies scaled to OO (sometimes referred to as HO/OO).

Most popular between 1935 and 1955, American OO scale model trains have been manufactured by Lionel, Scale-Craft, Nason Railways, and many others. British OO models have been made by makers like Hornby, Bachmann, and Airfix. They are incompatable with classic American OO models but compatable with HO models.

To add somewhat to the confusion, there are actually three types of 1/76 scale OO models in England. Most common by far are the previously mentioned HO/OO models. An older version of the corrected gauge is EM gauge, which uses 18mm gauge track. And a more recent version is P4 which operates on exact OO gauge track with no compromises in terms of wheels (i.e., they are fine scale models).