Yogicly speaking, Ommmm
is the sound the Universe first made whaen it came into being. Omm would most likely be used by a monk on top of a mountain because he is submerged in the never ending Universe. However, Guru Nanak Ji taught us that we need not be renunciates to find God, that the life of house holder can meditate as well. So we have ong, which is the creative aspect of the self. In this society, we create income and children and homes etc...so we are honoring the creative ascpect of the universe. That is the difference between Ong and Omm, as I have come to understand it.
Sat Naam-Hukum Kaur-