SB 6.5.26
tad-upasparsanad eva
vinirdhuta-malasayah
japanto brahma paramam
tepus tatra mahat tapah
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
At Narayana-saras, the second group of sons performed penances in the same way as the first. They bathed in the holy water, and by its touch all the dirty material desires in their hearts were cleansed away. They murmured mantras beginning with omkara and underwent a severe course of austerities.
Purport by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
Every Vedic mantra is called brahma because each mantra is preceded by the brahmaksara (aum or omkara). For example, om namo bhagavate vasudevaya. Lord Krsna says in Bhagavad-gita (7.8), pranavah sarva-vedesu: “In all the Vedic mantras, I am represented by pranava, or omkara.” Thus chanting of the Vedic mantras beginning with omkara is directly chanting of Krsna’s name. There is no difference. Whether one chants omkara or addresses the Lord as “Krsna,” the meaning is the same, but Sri Caitanya Mahaprabhu has recommended that in this age one chant the Hare Krsna mantra (harer nama eva kevalam). Although there is no difference between Hare Krsna and the Vedic mantras beginning with omkara, Sri Caitanya Mahaprabhu, the leader of the spiritual movement for this age, has recommended that one chant Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare.