"Om" is the universal sound, symbolizing the Infinite. It is like saying "Amen"...and derives from the same basic source of consciousness.

"Shanti" means "peace". As such it is very similar to the Hebrew word "shalom" or the Arab word "salaam"...again because it derives from the same basic source of consciousness. Hindu prayers normally end by saying, "Om Shanti"...which is the same as to say "Amen, Peace".