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.
American Seal & Packing - General Design and Gland Dimensions
O-Rings Viton O-Ring|FKM O-Ring|O-Ring|Hydraulic Seal|Rubber
ORing eHandbook
Determining the Right O-ring Dimensions for your Application
What is an O Ring - Selection, Material and Storage : Barnwell
O Rings - an overview
Seal & Design Manufacturer & Distributor of Custom Sealing Products
Static O-Ring Groove Design – Rod and Piston Glands
TWCC 382 PCS SAE FKM Viton O Ring Assortment Kit 30 Sizes Fluorocarbon Oring for Aviation Car Engines Cylinders Hydraulic Plumbing Various Industrial Chemical High Temperature and Oil Resistance 75A: : Industrial
Groove Design: Quad-Ring® Seals - Minnesota Rubber & Plastics
O-Ring Groove Design Standard AS568B • Seal & Design, Inc
O Ring, Square Cut O Ring, Quad Ring
Seal & Design Manufacturer & Distributor of Custom Sealing Products
O-Ring: What Is It? How Is It Made? Types Of & Common Uses
What you Need to Know About Designing Axial Seals