As per the Vedas, when this earth was created by God, first word yielded and it was Om. As per vedas, it is braham mantra. Gita is also summary of Vedas. In Gita, Shri Krishna has given the word Om for remembering. It will give the freedom from bodies and from bith and death. So, we all should speak and remember the God through Om.
Lecture by Dr. Mathura Das Sawtanter (M.A., M.ED and PhD), More about him at http://svtuition.com/dr-mathura-das/