Regarding Omkar, you may know that Om is the impersonal sound form of Krishna. Those who are unable to utter immediately the Sweet Name of Krishna, they are recommended to chant Om. Just like a child is taught to make a particular type of sound, and thus he learns the name of father, mother and other relatives. That is the beginning. So in the beginning, Om is the right vibration of transcendental sound, and the meaning is to address the Supreme Lord. Our business is directly with the Supreme Lord. Impersonal feature of the Lord is just like the illumination of the lamp. The lamp is the more important thing. You will gradually understand all these features of the Absolute Truth if you read Bhagavad-gita and adhere to the regulative principles.
Letter to Harold Torf, 15 November, 1969