The oxygen index (OI) represents the amount of oxygen relative to the amount of organic carbon present in a sample. The S3 curve of a Rock-Eval analysis can help us determine the total amount of oxygen present in a sample according to the following formula: | The oxygen index (OI) represents the amount of oxygen relative to the amount of organic carbon present in a sample. The S3 curve of a Rock-Eval analysis can help us determine the total amount of oxygen present in a sample according to the following formula: |