Om is the primordial sound, the highest vibratory frequency in the cosmic at all times.  This is the ultimate mantra, but it is not easy to pronounce correctly and is often pronounced wrong. Om is the only sound, everything we hear is a variation in terms of tone, but the sound stays the same.
Om opens you up, opens your heart to yourself.  With the sound of om, you can feel doors opening inside yourself and emotions coming out.  These can be both positive (happiness) and negative (pain). Om opens you up to the best side of yourself.
Om is very powerful and can give you tremendous benefits. Om is the cosmic energy, it is magenta, silver and gold.