Om is the name of the Universe, the sound of life and one of the many names for the One who created all things. People say God's name while meditating because it harmonizes them with the Universe...with God. That name is 'Om'.
If you think that meditation is something 'pegan' and/or satanic, think again. It's nothing New Age, or something created by people for control.
I believe that when Jesus 'prayed' he wasn't folding his hands and kneeling on his knees. He was meditating. he spoke to God in that way.
Remember that 'Om' isn't the only name for the All Father. He, like the All Mother, have many names. Different cultures, species and planets all have their own names for the All Makers, but Om is one of them.