An oil seal has three sizes, ID (inside diameter), OD (outside diameter), W (width). Normally, a standard oil seal size will be given, ID 20 x OD 30 x W 7 mm. Some customers and users who are not professional also call the width H/T (height/thickness).
Metric oil seals are more straightforward because the size is already stamped on the seal. For example, 20x 30x 7 means that the inside of the oil seal is 20mm, the outside is 30mm and the width is 7mm.