He has been building his repertoire with Wagner (Gurnemanz, Hagen, Hunding, Fafner, Pogner, Heinrich) and other leading bass roles such as Osmin/The Abduction from the Seraglio, Timur/Turandot, Sparafucile/Rigoletto, Sarastro/The Magic Flute, Kaspar/Der Freischütz, the Doctor/Wozzeck, Rocco/Fidelio, and Basilio/The Barber of Seville in the ensembles of the Mainz, Ulm, Detmold, Bern, and Mannheim opera companies.

Rúni Brattaberg was engaged at the Theater Ulm from the season 2006/2007 to 2007/2008, where he debuted as a doctor in Wozzeck under the baton of James Allen Gähres, in a production directed by Matthias Kaiser.

From 2009 to 2011 he was engaged at the National Theater Mannheim performing Gurnemanz, Hagen, Hunding, Fafner, Veit Pogner, König Heinrich, Basilio, Timur, Sparafucile, Sarastro, Baron Ochs, and Osmin.

[7] The collaboration between the two artists continued, and along with Faroese singer Eivør Pálsdóttir, a performance and recording of two works, conducted by Bryars, was realized in the Faroe Islands in 2011.

[13][14] From March to May 2014, he sang the role of Baron Ochs in "Der Rosenkavalier" at Malmö Opera, and in July of the same year he performed in Auckland, New Zealand, appearing as König Marke in Wagner's "Tristan und Isolde".