O’Rourke is part of the High Wood Health consortium with Serco who will provide estate management services.
NHS Dumfries and Galloway confirmed the winner yesterday.
The race was narrowed down to O’Rourke and Balfour last November when Skanska dropped out of the running.
Balfour landed a £1.1m access road deal for the site in June to upgrade the Garroch roundabout on the A75.
Jeff Ace, Chief Executive for NHS Dumfries and Galloway said: “This is the major milestone that we have been working towards for months now, along with two bidders who have produced the final designs for the kind of facility that we need so that our staff can deliver the best care possible for our patients.”
Dennis O’Keeffe, Project Director said: “Following an intense period of competitive dialogue and evaluation, we are pleased to announce High Wood Health as our preferred bidder.
“Their proposed solution is one that brings to bear a consortium of designers and contractors with considerable expertise and resources to fulfil our expectations.”