How are the major diameter of rubber O Rings specified -- outer, mean or inside diameter ?

Using these suppliers you can spec either inner or outer diameters or even cross section

[Edited on 11/10/15 by Rich J]

Generally you spec the ID and the section diameter. Google the IS Clarendon catalogue. It has just about every size O-Ring ever made listed, if it's not in there you probably can't buy it anywhere.