ID doesn't really matter. The closer the ID is to the OD number the thinner the walls will be. I have never had to think about ID when purchasing polypro. Who are you ordering from?
Are you ordering a hoop or tubing to make a hoop? The 5/8" OD is referring to the size of the tubing.
It sounds like you're asking about the size/diameter of the hoop itself... that's usually measured as the outer diameter (OD), but some companies like Astral Hoops measure inner diameter (ID). The difference depends on the size/OD of the tubing. So for example, a 36" OD hoop made of 1" OD tubing, is going to have an ID of 34".
ID refers to the wall thickness of the tubing, not necessarily the diameter of the hoop itself.
Are you ordering from someone specific or are you just doing research?