There are two main o-ring sizing types. Standard was developed for use in the USA, while the international community uses metric for their o-ring dimensions.
O-Ring Groove Design - Daemar Inc
eFunda: Introduction to O-Rings
How to Design an O-Ring Gland: Stretch
O-Ring Calculator Ceetak Sealing Solutions
JMSE, Free Full-Text
O Rings - an overview
O-rings, Quad Rings, Encapsulated Orings American & Metric Sized
Custom Metric O-Rings, Standard Metric O-Rings
O-Ring: What Is It? How Is It Made? Types Of & Common Uses