American OO is a unique gauge with a unique history. The British started making small models in the 1920s with a body scale of 1/76 but the track gauge of 5/8 inch. The appropriate scale for track of that width would be 1/87, so the bodies were over scale. They still make these HO/OO models widely today, and also models may be obtained in several versions of a correct gauge with the 1/76 bodies, including EM Gauge and Scalefour.
Models built in American OO are noticeably larger than HO but smaller than S gauge, which was developed later. American OO was produced in large commercial quantities by Lionel (1938-41) and Scale-Craft (1937-55), and in significant quantities before and after the war by companies such as Famoco, Nason Railways, Kemtron, Schorr, and many other firms, primarily between 1934 and 1955 with some production lingering on into the 1960s.
American OO had a very dedicated following before and after the war. The raw number of individuals peaked before WWII, but quite a number of people had gotten going and were builders who stuck with the scale. As a group, those “OOldtimers” are almost all deceased now.
As the scale lost popularity the OOldtimers had various tactics toward keeping active. There were clubs that met, especially the group(s) in New Jersey, and others corresponded by mail. An OO Speical Interest Group was started that still exists today. They kitbashed and scratchbuilt models. Some got very into scratchbuilding modern cars and converting HO models to use in OO in particular. Which is very possible in some instances as especially older production HO there were models that were produced that were overscale.
Today there is a newer generation of people following the scale. We all got into it long after the scale was popular, but the models are generally speaking pretty affordable and easy to work with in terms of restoration or rebuilding, and of course have a lot of character. Personally I enjoy the history of the scale and working with old and new models. It combines interests in modeling and collecting in a unique way.
So while the main source if you are looking to buy American OO is eBay, there are finds that come up and a lot of things can be modified for OO use from HO and Sn3. Our track gauge is the same as On3, so that track can be used in a pinch for operation along with wheelsets and drive parts. And of course there are British items that can be easily used in a North American context, such as modern 1/76 scale shipping containers.
In short American OO is not dead. It lost the battle of the gauges but has a following, especially from the Lionel collecting side of things. Much more on the history and operation of these classic models may be found in American OO Today, http://americanoo.blogspot.com/
This article was written this summer for the newsletter of the Model Railway Society of Ireland, and is presented here with permission of the editor. The illustration of the comparison of OO and HO gauges is from the October, 1939 issue of Miniature Railroading (presented previously in this article), and the photo is of my first OO car (Eastern) and a MDC HO boxcar, from this article.