If the copper tubing is of 'Nominal' size, it is the inside diameter (ID) that is used (1/2" is the Nominal size that might be used in your home for instance).
If it's a Refrigeration tubing, then the outside diameter (OD) is used.
Either way, they are basically identical with respect to dimensions (1/2" Nominal is 5/8" OD for instance).
Since different wall thickness makes the ID a slightly different dimension, you would likely be better off using the OD measurement.
However, since this is for an air compressor, the wall thickness is very likely 'Type K', which is the thickest of the normal off-the-shelf coper tubing.
Type M = Thin Wall (not made in Soft Copper configuration)
Type L = Medium Wall (made is Soft Copper configuration)
Type K = Thicker Wall (made is Soft Copper configuration)
So, just measure the OD of the tubing and that should put you on the right track.
Mike