Every Vedic mantra is called brahma because each mantra is preceded by the brahmaksara (aum or omkara). For example, om namo bhagavate vasudevaya. Lord Krishna 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 Krishna’s name. There is no difference. Whether one chants omkara or addresses the Lord as “Krishna,” the meaning is the same, but Sri Chaitanya Mahaprabhu has recommended that in this age one chant the Hare Krishna mantra (harer nama eva kevalam). Although there is no difference between Hare Krishna and the Vedic mantras beginning with omkara, Sri Chaitanya Mahaprabhu, the leader of the spiritual movement for this age, has recommended that one chant Hare Krishna, Hare Krishna, Krishna Krishna, Hare Hare/ Hare Rama, Hare Rama, Rama Rama, Hare Hare.
Source: A.C. Bhaktivedanta Swami Prabhupada (2014 edition), “Srimad Bhagavatam”, Sixth Canto, Chapter 05 – Text 26