OO Scale has an interesting history. The most popular model train scale in the United Kingdom, OO Scale (or OO Gauge) trains run on 16.5 mm gauge track, just like HO, but built to 1:76 scale to allow space for the motor and electrical equipment in the smaller dimensions of English rolling stock. Compare the real world LNER Mallard, fastest steam engine in the world, next to a Union Pacific Big Boy, and you'll see the dramatic difference in size.
Common manufacturers of UK OO Scale are Hornby, Heljan, Bachmann Branchline, Peco, and more.
To make things more interesting, in 1938 Lionel released a line of OO Scale trains. The rolling stock followed the same 1:76 scale, but ran on 3/4" (or 19 mm) track. Production ran until 1946, making OO Scale Lionel trains a rare item indeed.