From Adam Kilgore:
The Nationals traded slugger Michael Morse to the Seattle Mariners in a three-way trade that netted them right-hander A.J. Cole, the pitching prospect who last winter served as the centerpiece of the trade that brought Gio Gonzalez from the Oakland A’s to the Nationals. The Mariners sent catcher John Jaso to the Oakland to complete the deal.
So the Nats and M's re-acquire players and Oakland gets a good backup catcher.