O Holy Night by Adolphe Adam is arranged here by Peter Covino, Jr., for brass quartet, (trumpet, horn, trombone, tuba) and piano. This arrangement modulates everywhere but is not difficult. Order by mail here for $6.00 for score and parts. Free LDS choir sheet music, hymn arrangements, and piano and voice songs are available here.
It is also available for full orchestra and other combinations at this site. You can request a key.