Dear RoSpinner,
I wondered about that too, how to sing OM and inhale at the same time! This is when I was learning to do the Small Universe meditation. But I just kept doing the exercise along with the tape. It took me awhile to learn the sequence on that one. By the time I finally had it, I found I could do it without the tape, and it was easy to imagine the master's voice, singing OM, and MO.
For me, it is as Iam2 says, you say OM in your heart.
Best wishes,
Anita
