Tuning O-Ring Question LCS SK-19

I haven't torn the gun down yet, and need to order some o-rings. I apologize in advance if this is a stupid question, the only reference I can find online is the AoA video:

https://youtu.be/DFdnzURgTeU?t=364

When Don says use a 5 x 1.5 mm o-ring, is he referencing O.D. or I.D?

Thanks.