Chief Superintendent Steve Fulcher, leading the inquiry, said: "If there is a second body, we believe it relates to an incident some years ago and it is an adult.
"Wiltshire only has one outstanding missing adult case and that is from some years ago."
It is understood teams investigating the murder of Melanie Hall, 25, in 1996, are to liaise with officers in Wiltshire, while the 2001 disappearance of Tina Pryer in Trowbridge is also being looked into again.
Miss O'Callaghan's body was discovered yesterday near the famous "White Horse" landmark in rural Oxfordshire.
Could this be the girl missing since 2001 that the Police are searching for.