The OM symbol symbolizes the first frequency that created the universe.
Sound is frequency and the whole universe is made out of frequencies. In Judaism, the Creator speaks. In Christianity, there is "the word" and in Buddhism, there is the Om. The pronunciation of the Om creates the sound of movement and rest, which again means a frequency. It is interesting to see the relation between the Om and the word "mother" in different languages, the first word a baby pronounces.
The symbol on top of the OM also has a meaning -it symbolizes the start of creation from the vast emptiness.