The OD is the Outer Diameter of the tubing, so how wide the whole tube is.
On the Primochill tubing we were talking about above, the OD 5/8".
The barbs I recommended are 3/4" OD, so slight bigger than 5/8". But considering that you will be putting the tubing on barbs slight too big, the OD is going to become a bit bigger because of it. You could use 5/8" OD clamps, but expect it to be a bit of an effort to get them around the barb.