The most common O-ring sizes are based on the international standards listed below.  The size range which is available in the greatest range of sizes, materials and hardness is the BS1806/AS568B.

Standard Description Cross-sections
BS 1806 British standard for inch series O-rings. This standard has the same development roots as the AS-568B standard, and it incorporates some additional sizes. 1.02, 1.27, 1.53, 1.78, 2.62, 3.53, 5.34, 6.99
SAE AS568B American standard for O-rings. AS-568 is the Aerospace Standard (AS) as well as the SAE standard 1.02, 1.27, 1.53, 1.78, 2.62, 3.53, 5.34, 6.99
BS 4518 British standard for metric series O-rings. BS 4518. 1.6, 2.4, 3.0, 5.7, 8.4
SMS 1586 Swedish metric standard 1.6, 2.4, 3.0, 5.7, 8.4
DIN 3771-1 German metric standard for fluid-sealing O-rings. 1.80, 2.65, 3.55, 5.30, 7.00
ISO 3601-1 International standard for metric and inch series O-rings. The second edition includes the requirements for Aerospace (AS) applications. 1.80, 2.65, 3.55, 5.30, 7.00
JIS B2401 Japanese industrial standard for metric series O-rings. This standard now includes the second edition of ISO 3601-1. 1.90, 2.40, 3.10, 3.50, 5.70, 8.40
NF T47-501 French standard for O-rings. This standard is similar to the ISO 3601 and the DIN 3771 standards with some unique class designations. 1.80, 2.65, 3.55, 5.30, 7.00

Please refer to Downloads page for pdf copies of O-ring charts.