The Olum is a hypothetical organ which exists outside of the body, and is part of the digestive system. Its role in the body is to store and dispense potentia.
Magic has long been linked to the appetite of the user, but it was not until 82 GD that this was experimentally proven. Dissection of living bodies kept alive with magic was once a commonplace method to obtain medical knowledge, and through this method it was discovered that no physical organ within the body houses potentia, and that the source of it must be outside of the body instead. It was also proven, however, that even when a magic user had an abundance of potentia, their spells weakened noticeably when they became hungry. The name “olum” was given to the hypothetical organ where potentia is stored physically.
Being that no physical olum could ever be dissected, the structure of the olum could only be ascertained through scrying.