When I was young, I used to see the OM symbol in Hindu temples. I used to wonder at its meaning. It is only later that I learnt that OM is clearly explained in Mandukya Upanishad (MU). MU is one of the ancient classical Upanishads commented upon by Acharya Shankara. It comprises of only 12 shlokas (verses). It is also one of the most fascinating for reasons that will become clear as we go through the short text. I found, however, that the significance of OM was not clear to me even after going through MU. I fully understood only after reading Vivekananda and Ramakrishna on OM. I will first of all go through the text of MU and then discuss the significance of OM as explained by Vivekananda and Ramakrishna.