In the Bhagavad-gita, this omkar is mentioned as being the sound representation of Krishna, the Supreme Personality of Godhead. Similarly the Hare Krishna Mantra is also the sound representation of Krishna. So, practically we do not make any distinction between the omkar and Hare Krishna but the spiritual path is to follow the footprints of the path of the great acaryas. Lord Caitanya encouraged the Sankirtana Movement, or the chanting of Hare Krishna, is the means for God realization for this age and it is in pursuance of the revealed scriptures.
Letter to Mr. Chagan Govinda, 17 December, 1968