Born in Mödling, Rösner studied at the University of Music and Performing Arts Vienna and attended master classes with Ilya Musin, Myung-Whun Chung and Hans Graf.
He received decisive musical impulses as répétiteur of the Wiener Singverein (direction: Johannes Prinz), where he worked with conductors such as Riccardo Muti, Mariss Jansons, Georges Prêtre, Roger Norrington and Fabio Luisi.
In 1998, at Luisi's invitation, Rösner became assistant with the Orchestre de la Suisse Romande.
From 2000 to 2004 Rösner was "Chef associé" at the Orchestre National de Bordeaux alongside Hans Graf.
In addition, he has given world premieres of works by Philippe Fénelon, Rudolf Kelterborn, Thomas Daniel Schlee, Jean-Luc Darbellay, Jost Meier, Gerd Kühr, Wolfram Wagner and Edward Rushton.