If it was the “traditional” Lionel three-rail trains, it was O scale then and now.
The name “O gauge” goes back almost 100 years now, Marklin trains were made in gauges 1-4 with gauge 1 (now used by LBG “G” scale trains) being the smallest. When Marklin came out with a smaller (1-1/4") gauge they continued using numbers and called it “0 gauge” as in “zero gauge”. At least in English speaking countries “zero” became called “Oh” gauge.
HO is “Half-O”, track gauge 1/2 the size of O track.
Lionel and other manufacturers still make a broad range of O gauge trains. Some are built to 3/16th" scale like the toy trains you probably had in the sixties, but now many items are made in correct 1/4" scale size, the correct ratio (1:48, 1/4" = 1 foot) for O gauge scale models.
Well OK 1:48 is a little off but it’s the most common scale ratio used in the US for O trains.
Yes, we still call our larger trains O gauge. G gauge has become popular and is even larger. But O gauge is the most popular of the larger gauges. Lionel and Marx were popular in the 60’s. Lionel is still available today. Marx is gone but some of the tooling got absorbed by K-Line which is now owned by Lionel. We also got Atlas, MTH, Weaver, Williams, 3rd Rail, etc. But Atlas, Lionel, and MTH are probably most popular.
Welcome Welcome Welcome Let me say you have made a good move. I do O gauge/scale and I like to call it 3rail Scale and there is alot to pick from in O. Atlas O, Lionel ,Weaver,to name a few. you can find them on the web. HO is nice but O… is the way to go, now you will find more forms like this one on the web with good info also so take your time and Happy Rail Roading.