The Oracle is a being who is worshipped, along with other gods, by the Igbo. It is seen as a source of wisdom and knowledge for those who are willing to approach and ask. No one ever sees the Oracle and it is made separate from the human who actually acts as the voice. The Oracle's advice is considered to be sacred and disobeying it is said to bring negative consequences.
The Igbo people would consult the Oracle in all types of situations, whether it be personal ones, like marriage or issues in a family, or issues that might affect the whole tribe.
The Igbo are not alone in having the concept of an Oracle, it is common among a great many ancient cultures. Though it may have different names in different cultures, in ancient Greek culture it was also referred to as an Oracle and has been memorialized in a large number of plays and Greek texts, as a similar source of light and wisdom that regular people could consult when necessary.