Osborne will be responsible for the maintenance of 5,400 council-owned homes as well as several council buildings, including the Grade II listed Guildhall.
The work varies from fixing a dripping tap to major structural repairs.
One of the challenges is to prepare empty properties for use quickly so that the council can house new residents, and reduce re-letting times and tenant waiting lists.
Nick Sterling, managing director of property services for Osborne, said: “At Osborne our aim is to deliver ‘right first time’ repairs, quality workmanship, customer-focussed service and value for money.
“We hope that the residents in Winchester will benefit from our UK wide experience of making sure that we deliver the best possible service to the people we look after.”