In answer to John West's question about is there an OOn3 , try the following website :
[
www.backwoodsminiatures.com]
Has listing for OOn3, OOn9 and others.
OO and HO are very similar. OO is 4 Millimeters to the foot scale , HO is 3.5 Millimeters to the foot scale . OO scale locomotives run on HO scale track.! So they actually are Narrow Gauge models . HO scale track is 16.5 Millimeters wide which scales out to 49.5 inches wide in OO scale not the real 58.5 inches for standard gauge track.
I have a Hornby OO scale Flying Scotsman sitting here as I type this . The only OO scale model I own.
OO came about because the motors of the day wouldn't fit very well in the English locomotive bodies in the years after WW2.
Charles M