Sian is described as white, 5ft 3in tall, medium build with brown, straight hair and green eyes. She was wearing a grey dress, black bolero-style jacket, dark coloured tights and flat boots. She was carrying a dark handbag which had a beige flower attached.
Officers said her being out of contact was totally out of character.
More than 60 police officers are involved in the search with officers continuing to patrol Old Town today and visiting businesses in the area.
Det Sup Steve Fulcher, who is leading the team, appealed to Miss O’Callaghan to contact her family or the police.
He added: “Today there will be further police activity including in the High Street in Old Town. I’d like to reassure the public that we are conducting a thorough investigation into Sian’s disappearance and that we have a team of highly trained officers working on the inquiry.
“In particular I am trying to establish Sian’s movements between 3.15am and 4am on Saturday morning in the area of Savernake forest.
“I would like anyone who saw her or any vehicles, especially in the area of Savernake forest where there are likely to have been only a small number of vehicles at that time of night, to make contact with police.”
