In all cases I am aware, tubing is measured on the OD. Where it gets complicated is there are several wall thicknesses for each tubing size. For example for 3/8", you can have 0.020, 0.028, 0.036, 0.048, 0.065, etc. wall thicknesses. The easy way to remember this is to make a 3/8" compression fitting work, the OD is the critical dimension.
