Detective Superintendent Steve Fulcher said the team from Wiltshire Police had been told they were those of a woman abducted in Swindon between 2003 and 2005.
He also revealed that an examination Sian's body revealed she had not been sexually assaulted.
News that the remains of a second person had been found came hours after police were granted a further extension by magistrates in Swindon to hold the man being questioned in connection with Sian's death.
They have until 3.15am on Monday to question him further, before they must either charge or release him. He was arrested on Thursday after police swooped on a taxi rank at an Asda supermarket in Swindon.
A green Toyota Avensis with taxi markings was also taken from the scene.
Police have been digging on Baxter's Farm, in the Gloucestershire Cotswolds 17 miles from where Sian's body was discovered, since late on Thursday.
The body of Miss O'Callaghan was recovered from an area 15 miles east of Swindon, close to the Uffington White Horse. It is believed her body was dumped there several days after she went missing, as the media coverage and public hunt for her intensified.
Wiltshire Police said hundreds of floral tributes to Sian, which were placed outside the Fuju nightclub where she had spent the evening before her disappearance with friends, will be removed tonight to allow it to reopen.
Police search
Det Supt Fulcher said on Friday he had been taken by the suspect to the location where Miss O'Callaghan's body was found and where the search had begun for another victim.
The farm at the centre of the search lies in rolling countryside a couple of miles south of the Cotswold Wildlife Park, outside the small market town of Burford.
Specialist dog teams have been helping the search after mobile phone technology pinpointed several areas of interest in 4,500-acre Savernake Forest.
Police seized a green taxi which was seen early last Saturday when Miss O'Callaghan disappeared after leaving the nightclub in her home town of Swindon.
Miss O'Callaghan, an office administrator, was caught on CCTV leaving the nightclub, in Swindon's Old Town area, after an evening out with girl friends.
She lived in a flat just half a mile away with her boyfriend Kevin Reape, 25.
She had sent a text message to her partner on the night of her disappearance, saying: "Where are you? x."
Police revealed earlier this week that phone records put Miss O'Callaghan's mobile somewhere in the forest 34 minutes after she left Suju nightclub at 2.52am on Saturday.
Miss O'Callaghan's family thanked members of the public who helped in the search for "our beautiful girl".
http://www.channel4.com/news/police-probing-sian-murder-find-second-body
