Officers began searching last Thursday after they discovered the body of 22-year-old Miss O'Callaghan near Uffington, in Oxfordshire, about 15 miles away.
Miss O'Callaghan, an office administrator, went missing after leaving a nightclub in Swindon in the early hours of March 19.
Her boyfriend, Kevin Reape, raised the alarm after she failed to arrive at their home half a mile away, and more than 400 volunteers joined the search. Yesterday, officers searched fields near Shrivenham to try to find Miss O'Callaghan's missing DKNY watch, car key and house key.
Chris Halliwell, 47, a taxi driver, has been charged with her murder. He is due to appear at Bristol Crown Court today for a bail hearing.
Police had been hoping to identify the second woman this week. In a statement issued yesterday, Wiltshire Police said: "Forensic investigations to establish the identity of this young woman are ongoing using DNA and the speed of this identification process relies on whether or not we have her DNA on a database.
"If we don't get a DNA match, further investigations will need to be carried out, which may take some time."
Forensic officers yesterday completed their examination of the place where the woman's remains were found.
Flowers and a wooden cross were left at the scene, with a handwritten note which read: "To an unknown lady... Now you can rest in peace. Wiltshire, Thames Valley Police and Avon and Somerset Police."