There is no reliable visual inspection method to determine O-ring material composition, only specialized materials testing equipment can make that determination. Acrylonitrile butadiene rubber elastomers (aka NBR, aka Nitrile, aka Buna-N) are considered to have good resistance to oxygen with a 15 year shelf life. Ethylene Propylene Diene Monomer (aka EPDM) and Fluorocarbon elastomers (aka FKM, aka FPM, aka Viton™) are considered to have excellent resistance to oxygen, with an essentially unlimited shelf life. To ensure specific composition, O-ring material must be specified at time of purchase and then segregated from other O-rings.

There are no O-ring color coding standards and thus color or appearance is not an indication of the material or quality. O-rings are available in nearly any color, although most commonly seen in black regardless of material. Even though most O-rings are black, the loose O-rings sold by Dive Gear Express are oxygen compatible and usually brown, green or purple; simply to make them easily distinguishable from more common NBR O-rings which are usually black. However, oxygen compatible O-rings inside equipment (valves, regulators, hoses, service kits, etc.) are almost always black. O-ring materials also can not be distinguished from each other by their surface appearance, more a function of the manufacturer than the material.

Just to be clear, the COLOR and SURFACE APPEARANCE of the O-ring is MEANINGLESS regarding their compatibility with oxygen.