Organic content (OC) is an important characteristics when it comes to organic soil, especially peat. Generally, the OC is mainly for classification purposes and can be referred to previous studies that has been done.
Higher OC means less mineral content (Mineral content = 100% - OC). Usually high OC relates to very problematic soil, having very high compressibility and very low strength.
Concerning method, from reference to ASTM and BS, both outlined the ignition of soil sample in a muffle furnace at 440 C until constant mass is achieved. (The weight of ash as the mineral content) So, i don't think there wil be much trouble with standards, as these are the 2 of the most widely used standard.