OD means outside diameter. A 5/8-inch OD grommet has a nominal outside diameter of 15.875 mm, usually measured across the outer flange for a conventional round profile.
That value is not necessarily the required mounting-hole size. Check the groove diameter, panel-thickness range and internal passage on the drawing. Include cable and connector clearance.
A grommet helps reduce abrasion at the opening but does not replace grounding or other electrical protection. Select the compound for the environment and verify sealing or vibration performance separately when those functions are required.
