South African-born O'Brien, who has experience in England with Blackburn Rovers, has today completed a three-year deal.
The signing is apparently quite a coop from Paul Sturrock as some big clubs in the SPL were also chasing the 24-year-old's signature.
O'Brien is more of a creative midfielder than a tackler and joins Graham Coughlan, Drissa Diallo and John Hills as new faces at Hillsborough this summer.
Another new face is striker Barry Corr - signed from Leeds last season - expect to see him in a few pre-season games before the start of the campaign.
Paul Sturrock is also handing a trial to Hungarian utility man Zoltan Vasas, who plays for Nyiregyhaza in his homeland.
He operates on the left-hand side but cannot play in any trial games because he is still contracted.