The vocals are courtesy of bluegrass legend Ralph Stanley, who became a name with his late brother Carter back in the 40's and 50's. When you hear the traditional 'Angel Band' over the closing credits of O Brother that's the Stanley Brothers.
I'm a fan, needless to say. I read one person describe his voice as having a "high lonesome" sound to it. I think that's pretty apt. His duet albums with various other artists were also great as well.