Yes there is a difference between OO & HO scale, they are different sizes, they just run on the same track.
basicaly "OO" = 1/76 scale & HO = 1/87 scale. OO is pronounced Double O
But it gets a bit more complicated.
To start with OO & HO are not actually scales, they refer to the "Gauge" of the railway track that the trains run on.
What does GAUGE mean.
GAUGE is the distance between the steel rails of the track.
HO gauge track is 16.5mm wide.
What does SCALE mean.
The 'Scale' is the ratio of how the size relates to reality. HO = 1/87 scale OO = 1/76 scale
Examples of how scale works ;-
1/100 scale a person that is 1.68 metres tall (5' 6") would be 1.68 cm or 16.8 mm high
1/87 scale a person that is 1.68 metres tall (5' 6") would be 1.93 cm or 19.3 mm high
1/76 scale a person that is 1.68 metres tall (5' 6") would be 2.21 cm or 22.1 mm high
1/50 scale a person that is 1 68 metres tall (5' 6") would be 3.36 cm or 33.6 mm high
1/160 scale a person that is 1.68 metres tall (5' 6") would be 1.05 cm or 10.5 mm high
HO / OO THE FACTS
With model train sets there is a bit of a twist.
"HO" is 1/87 scale trains and accessories running on HO gauge tracks.
"OO" is 1/76 scale trains and accessories running on HO gauge tracks.
"OO" is a size of model trains made in England whereas "HO" is the most popular size used just about everywhere else in the World, such as Australia, Europe & the USA.
For some reason UK manufacturers, like Horny & Dapol, decided to market their stuff as HO/OO, which is where much of this confusion stems from. Many people have inherited, purchased or received a gift of a HORNBY Flying Scotsman train set or something similar. On the side of the box it says HO/OO, you go to the shop to buy HO gauge track & turnouts (intersections) to extend your layout to run your the train on and it all works really well. You can even buy a HO gauge train and run it on these same tracks, but when you put the two trains next to each other you will see that they are different heights. If you have Hornby or Dapol buildings or accessories, they look fine next to the OO gauge trains. But when you buy some HO scale figures or cars they look too small. You need to buy OO or 1/76 scale accessories such as buildings and figures to suit the size of these train and carraiges. 1/76 scale is very popular with Diecast model cars & trucks.
Then to add to the confusion there is 1/100 scale, 1/72 scale and 1/64 scale as well. 1/100 scale is often used for Architectural models, 1/72 is the popular scale for model Plane & tanks etc. with manufacturers like Airfix, Revell etc. 1/64 is approximately the scale of the MATCHBOX diecast cars.
To the uneducated eye there is not a lot of difference between these scales. There is about 3 to 6mm difference between the heights of the various scale figures. Trees, grass, plants, lights, road and many other accessories are interchangeable between the scales ( who is to say exactly how tall a tree or a lampost is and how wide a particular road should be).
Some of the other popular model train scales are;-
TT Scale = 1/120 12mm gauge track
N Scale = 1/160 rest of world or 1/150 in Japan 9mm gauge track
O scale = 1/48 USA 1/43.5 UK & France 1/45 Germany etc. 19mm gauge track
G scale = 1/120 to 1/25 scale 45mm gauge track
BROAD, NARROW AND STANDARD GAUGE
In the real world there are different gauges or widths of track, in Australia we have three main gauges.
Broad Gauge is the track width used in most parts of Victoria 1600mm.
Narrow Gauge is used in Queensland, Tas, WA and parts of SA 1067mm
Standard Gauge refers to the gauge used in NSW and later to connect NSW, Victoria and Queensland. 1435mm
To more accurately depict a Queensland Narrow gauge train set people have decided on the designation HOn3½ which refers to HO 1/87 scale trains etc. running on N gauge track which is 12mm apart. 3½ refers to the Narrow gauge of 3 ft 6 in = 1067mm
I hope this has helped, for more info please see