Oak Park Concert Chorale
Oakland Interfaith Gospel Choir
Oakland Jazz Choir
Oakland Symphony Chorus
Oberlin Choristers
Occidental Glee Club
Occidental-Foothill Master Chorale
Octarium
Ohio Boychoir
Ohlone Chamber Singers
Ojai Camerata
Ojai Community Chorus
Oklahoma Baptist Univ. Bison Glee Club
Oklahoma State University
Olentangy High School Choirs
Olympia Choral Society
Olympia Youth Chorus
Omaha Chamber Singers
Omaha Symphonic Chorus
On Call
One Accord
One Spirit Choir
One Voice Mixed Chorus
One Voice Outreach Chorus
Onyx Opera Atlanta, Inc.
Opus 7 Vocal Ensemble
Opus Q - The East Bay Men's Chorale
Orange County Catholic Chorale
Orange County Classic Choral Soceity
Orange County Women's Chorus
Orange Park Chorale
Oratorio Chorale
Oratorio Society of Minnesota
Oratorio Society of New York
Oratorio Society of Queens
Oregon Children's Choir
Oregon Children's Choir and Youth Chorale
Oregon Festival Choirs
Oregon Repertory Singers
Oriana Singers, Chicago
Orlando Chorale
Orlando XIII
Orleans Chamber Singers
Orpheus Chamber Singers
Orpheus Club
Orpheus Male Chorus
Oshkosh Chamber Singers
Our Lady of Lourdes Children
Out Of The Blue