The oil hole plug(s) are used to block one or both of the axial oil holes in a stud type cam follower; If you will be relubricating a bearing, a grease fitting is inserted in one of the axial oil holes and an oil hole plug is put in the other; When the bearing is lubricated, the oil hole plug prevents grease from flowing out the unused oil hole and forces the grease to go to the rolling elements; If you are not relubricating a bearing, the oil hole is used to help prevent contamination from entering the bearing; Some cam followers do not have an axial oil hole at both ends of the bearing, thus an oil hole plug is not always required.