Oracles are something or someone that can foresee the future or scry. The main examples in the books are the Singing Sisters and the Totem Wall. It is unknown how the Totem Wall uses it's Oracle power, as it is only briefly mentioned to have oracle power by the Singing Sisters. We do know that the Singing Sisters use singing to obtain oracle knowledge, which as shown in the books, can be physically painful if trying to obtain deeply guarded knowledge.

Source

• Keys to the Demon Prison (Fablehaven #5)