Wiltshire police said a DNA match identified remains discovered in a field in Eastleach, Gloucestershire, belonging to 29-year-old Becky Godden-Edwards of Swindon.
Police said she was reported missing in 2007 after becoming disconnected from her family. Detectives said Becky and Sian did not know each other and that it was still not known how she met her death.
Christopher Halliwell, 47, also of Swindon, has been charged with the murder of O'Callaghan. He led officers to the Cotswold beauty spot where Godden-Edwards's remains were found.
"Part of these enquiries will be to speak to Chris Halliwell and we will then make a decision on any appropriate steps to take," Wiltshire police said in a statement.
O'Callaghan vanished after leaving a nightclub two weeks ago. Her disappearance prompted a massive police hunt, backed up by hundreds of locals. Her body was later found in Uffington, about 12 miles east of Swindon.
(Reporting by Stefano Ambrogi; editing by Michael Holden)