Many buddhist mantras have Om/Aum in starting of it. In buddhism, esp. theravada, what is the meaning and "significance" of "Om"?
(note:although I am open to mahayana,vajrayana interpretations too as an "additional" input)
There is no OM or AUM in Theravada to my knowledge. It's found only in Mahayana and Vajrayana.
The 14th Dalai Lama explained here the meaning of OM in "OM MANI PADME HUM" as:
The first, OM, is composed of three pure letters, A, U, and M. These symbolize the practitioner's impure body, speech, and mind; they also symbolize the pure exalted body, speech and mind of a Buddha.