O-1 is an AISI, American Iron and Steel Institute, code. Those are codes that indicate the steels quench medium, such as O for oil or W for water, use, such as D for die steel, or content, such as L for low alloy or M for molybdenum alloy. The codes that are generally used that are just four or five numbers, such as 1084 or 52100 are SAE codes, which stands for Society of Automotive Engineers. The last two or three numbers in the series indicate the carbon content. The first two are going to indicate that alloy so with 52100 the 5 indicated that it's a chromium alloy. The 2 indicates that it's chromium content is ~2% and the 100 indicates ~100 points of carbon, or about 1%.
Now there are other code systems out there. The A36 that you can pick up at the hardware stores is not air quenching. As a matter fact, the stuff will hardly harden at all. It has to do with physical characteristics of the steel like the stresses it will take.
Doug