In Book X of the Odyssey, Circe takes an oath that she is not devising ill against Odysseus. She undertakes this oath when Odysseus asks it of her following what he had been told to do by the messenger god Hermes. The circumstance of Circe's oath is part of a database on ancient oaths from the University of Nottingham, Oath in Archaic and Classical Greece Project.

The Oath in Archaic and Classical Greece Project provides 3 simultaneous criteria for an oath.

1. Someone must make a declaration,
2. implicitly or explicitly, a superhuman power is asked to bear witness,
3. and an implicit or explicit curse is called down upon the declarer of the oath if it is false.

In the case of Circe's oath, she doesn't explicitly declare a witness/guarantor, nor does she name a curse that she will suffer if she is lying, but she does, nevertheless, keep her promise.

Odyssey Book X Summary