It all depends upon how are you using OM Chanting meditation. The vibrations of Ooommm start from the lowest part of the spine and rise up with mmmm and fades away into the silence mingling with the cosmic vibration. If you are chanting OM for physical wellness it is suggested that you elongate the OOO part. and if you are looking for calming mind and emotional wellness elongating mmmm part can help a lot. Hope we answered your question.. Blessings and Peace all the way … OM.. OM .. OM