OO is approx 1:76, and actually dates back to the 1920's. Big in the UK. Lionel actually had a line of OO in the late '30's / early '40's, but it never really caught on in the US. Surprisingly, as Lionel's mainstream O began to decline in the tail of the 1950's, the smaller HO gained great popularity. In a way, Lionel was ahead of it's time with it's early OO run, but perhaps too early. It never really saw the HO birth on it's horizons.So I guess OO and O are completely different.