Mani Mantra is not only Tibetan Buddhist canon. In fact this mantra is within all Mahayana sutra canon. In chinese Mahayana canon, Om Mani Padme Hum also mentioned.
The following is the explanation of this Mantra in details:
The Mantra Om Mani Padme Hum
--------------------------------------------------------------------------------
The mantra originated in India; as it moved from India into Tibet, the pronunciation changed because some of the sounds in the Indian Sanskrit language were hard for Tibetans to pronounce.
Sanskrit : Om Mani Padma Hum (mantra of Avalokiteshvara)
Tibetan : Om Mani Peme Hung (mantra of Chenrezig)
The Common Mani Scripts
The mantra Om Mani Padme Hum is found written in two different ways in (and on) Mani wheels and on jewelry, etc.: in the ancient Indian Ranjana script and in Tibetan script:
Tibetan Script:

Sanskrit Script:

"There is not a single aspect of the eighty-four thousand sections of the Buddha's teachings which is not contained in Avalokiteshvara's six syllable mantra "Om Mani Padme Hum", and as such the qualities of the "mani" are praised again and again in the Sutras and Tantras.... Whether happy or sad, if we take the "mani" as our refuge, Chenrezig will never forsake us, spontaneous devotion will arise in our minds and the Great Vehicle will effortlessly be realized."
Dilgo Khyentse Rinpoche
-- Heart Treasure of the Enlightened Ones