The mandukpenita prescribes o. What it says, i'll tell you in brief and then conclude. The word has three syllables, a, the sanscritt is often written as a, but it's not the english a. Then the next syllable is o, and the third one is m. Om, pronounced together, becomes om. Now what the upanisha teaches is do this. Think about yourself now and your waking world, the waker and the waking world, in your mind, makan. It's an exercise, mak an, association. And imagine your dreams, some kind of dream, whatever you dream when you forget the waking world