Why do we often chant Om in Yoga Classes?

"Yoga" or union is is concerned with the individual consciousness uniting with totality(Brahm).

Wherever our attention is placed, that is what exists for us personally in that moment. This is due to the fact that our true Self or Purusha, having no shape of its own, takes on the form of whatever it observes. When chanting Om our attention is placed on totality, all possibilities and all probabilities. We for that moment identify with idea of Brahm or totality. 

अहं ब्रह्मास्मि ।

 Aham Brahmasmi

"I am Totality"

-Brihadaranyaka Upanishad 1.4.10