The mantra “Om namo Bhagavate Vasudevaya” fascinated me greatly in my early days; it so delighted me that I had always a vision of Sri Krishna in my mind. I had a premonition that this body would pass away in its fortieth year, and I wanted to have a darshan of the Lord before that time. I fasted and practised devotion to Vasudeva incessantly; I read Sri Bhagavad Gita and Srimad Bhagavatam with great delight. Then when I read in the Gita “Jnani tu atmaiva me matam” (In my view, the Jnani is my own Self ) I was greatly delighted. This line of thought came to me: “While I have at hand Bhagavan Sri Ramana, who is Himself Vasudeva, why should I worship Vasudeva separately?” Be it noted that all this was in my early days before settling with Bhagavan at His Ashram. So I wanted one single mantra, a single worship (devata), and a single scripture, so that there might be no conflict of loyalties. Sri Ramana Paramatman became easily the God to worship, His collected works easily became the gospel; as for the mantra, it struck me intuitively that “Om namo Bhagavate Sri Ramanaya” might be an exact parallel to “Om namo Bhagavate vasudevaya.”
I counted the letters in this new mantra, and was very happy to find it also contained twelve letters; I told this all to Sri Bhagavan, and He gave the mantra His approval.
Advanced practisers (sadhaks) and thinkers may laugh at this and say: “Why do you need a mantra while the Ocean of Bliss is there to be immersed into directly?” I confess that in this I was trying to conform to the traditional method of practice (upasana) which forms one of the main elements in bhakti (devotion). Bhagavan has revealed His true nature as the All-Witness; yet there is the explicit injunction that Advaita must be only in the attitude and never be interpreted in outer action.
This is how the mantra first came out.