"ॐ" - the most powerful mantra.

Lord Shiva explains about his work and nature in Shivapurana's Vidyeshwarsamhita; He says that "ॐ" is the most auspicious mantra and it is his nature and character.

There are some videos also and it is proven that the sound of the universe is 'ॐ'.

Link - 

https://youtu.be/84VIZ6d0nUk

Omakar has 5 parts - अ, उ, म्, बिंदु dot •, & sound (half curve in the symbol ॐ)

Shiva says that remembrance of Omkar is the rememberance of Lord Shiva.

As Omkar has got five parts, Lord Shiva has also five major works which are unibersal and incessantly proven. These five works are - CREATION, COMPLIANCE  (UPBRINGING), DESTRUCTION, DISAPPEARANCE  (ANONYMOUSNESS), GRACE (BENIGNITY)

First 4 are the expanding factors of this world but fifth one is the cause for the salvation that is Nirvana or emancipation.

Creation exists in earth element. Compliance / upbringing exists in water element. Destruction exists in fire element. Disappearance exists in air (vayu) element. Grace exists in the sky (void - Ākāsh) element.

The whole world is made of five elements as well as all the forms, illustriousness and masculine-feminine clans are pervaded by ॐ.

Shiva & Shakti (the ultimate strength and the ultimate energy) are indicated in ॐ.

ॐ has brought vowels as well as consonants into the existence. Tripada Gayatri that is Gayatri Mantra also came into existence through ॐ.

Various mantras, Vedas came into existence.

Mantras (psalm/hymn/chant) are the proven factors for gaining the pleasures as well as salvation.

Not only spiritual and holy books but even science is proving the benefits of chanting ॐ.

There are enormous benefits of chanting ॐ, physical, psychologicaland spiritual. Most of the illnesses are caused due to imbalance of the five elements and chanting ॐ can balance these five elements as well as it provides energy.

Almost all the mantras start with it and become powerful.