The planned 350-bed complex is earmarked to be built on farmland off the Garroch roundabout at the western end of the town’s by-pass.
The hospital will be built under the Scottish Government’s non-profit distributing model and must still reach financial close before work can start.
O’Rourke is part of the High Wood Health consortium with Serco who will provide estate management services.
Jeff Ace, NHS Dumfries and Galloway’s chief executive, said: “This is a major milestone which allows us to work towards financial close at the start of the new year, and paves the way for the start of construction scheduled for spring 2015.
“We have come a long way from the initial site selection of the green field site, on the outskirts of Dumfries.”