OM is a vibration or mantra. It's usually done at the beginning or the end of a yoga class. This vibration is considered to have high spiritual and creative power and can really be chanted by anyone. It's a sound and a symbol rich in meaning and depth. When pronounced correctly it is actually AUM. Traditionally, these three letters embody the divine energy (Shakti) and it's 3 main characteristics: creation, preservation and liberation.
So why do we do it? Well, everything in the universe is constantly vibrating. The sound of OM typically vibrates at a frequency considered to be the same vibrational frequency found throughout everything in nature. So by chanting OM we are physically and symbolically acknowledging our connection to all other living beings, nature and the universe. Also, the vibration of OM has a physical effect on the body by slowing down the nervous system and calming the mind. This allows your blood pressure to decrease and your heart health to improve. Overall, beginning and ending with OM as a class helps connect us to our practice, postures and each other in a much deeper way.