Just wanting to point something out. These definitions are from Robert Thurman’s fantastic (really really fantastic) translation of the Tibetan Book of the Dead/Liberation through Understanding in the Between:
OM: This mantric syllable is called the “body-vajra of all Buddhas.” It invokes the power of the divine and universal, resonating with its omnipresence, and therefore occurs at the beginning of most other mantras.
HUM: This mantra syllable is known as the quntessential “mind-vajra” of all Buddhas, symbolizing the integration of the universal, absolute, and divine within the particular individual. Thus it often occurs at the end of mantras, signifying that the spiritual attainment, deity, or positive energy invoked has been integrated within the indivdiual. It may correspond in some respects to the Christian Amen that concludes all prayers.
The mantra Om Mane Padme Hum means something like: OM! The Jewel in the Lotus! HUM! The jewel in the lotus being a reference to compassion as the heart of reality, which is quite consonant with the understanding of Hundred-Armed Avalokitesvara or Kuan Yun (who some believe is actually patterned after the Virgin Mary from contact the East had with images of our Lady through the Silk Road caravans).