O Scale is twice the size and 4 times the volume of HO.
A good reference is if an HO Scale railroad built on a 4'x8' framework, the space needed to build the same railroad in O Scale will need 8'x16'. All dimensions (Height, Width, Length) are doubled be it rolling stock or structures.
It can be hard to imagine the space requirement's in your mind.
Pick up a pre-owned freight car or two to get the feel of O, that way the investment will be minimal. Look at them, poke at them, compare them to their HO counterparts, roll them back and forth on a section of track to become comfortable with them. That's what I did when I transitioned from HO to S Scale.
Rusty
