On Monday, September 16, from 7 to 8:30 p.m., a Kilgen Theatre Organ performance at the Oklahoma History Center will feature organist Brett Valliant. The theme for his appearance will be “The Great American Songbook,” when Valliant will perform popular all-time favorites. This is Valliant’s second appearance at the Oklahoma History Center. In July 2020, he was the guest organist in a performance dedicated to “Discovering the Unit Orchestra.” Inspired by the organ at church when he was just three…