The O Series will tend towards being more free, while retaining the ability to sing with the head voice. With the O series has the ability to play with both head and chest playing, but the color, and weight of sound is more towards a blending darker sound, when pushed the O series is still alive and clear.
The mouthpieces designed in this series are deeper than the V series which are designed to be a solo mouthpiece. The O stands for Orchestral and will provide you with a deeper voice, richer sound, more weight of sound, and a sound that will blend easier with your colleagues.
The below S series pieces are listed in order from largest to smallest cup diameter.