After phoning family and friends on Saturday morning Mr Reape raised the alarm at 0945 GMT.
BBC: Boyfriend's appeal for missing Swindon nightclub woman
Kevin Reape told a police news conference he wanted to tell the public how worried everyone was about her.
"This is a terrible time for all of us and we are praying for Sian's safe return, " Mr Reape said.
At a news conference on Monday, her boyfriend Kevin Reape, who shared a flat with Miss O'Callaghan, appealed for information to help find her.
BBC: Hundreds search forest for missing Sian O'Callaghan
Mr Reape said he had spent the day at the Cheltenham Races before returning home about 2230 GMT on Friday and going to bed.
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