Lord Krishna in Bhagavad Gita says unless human beings negate karma to zero one can never reach state of enlightenment followed by moksha salvation. Then how can chanting a mantra Om Namah Shivaya lead one to moksha salvation. Such summations did not exist in Hinduism religion nor there any shortcuts to gaining moksha salvation.
Everything on path of spirituality as detailed in Bhagavad Gita is crystal clear. We just have to follow the path. As nothing in Bhagavad Gita can be understood literally, our indulgence in contemplation (chintan) is the only way out.
No mantra including Om Namah Shivaya leads mankind towards state of enlightenment followed by moksha salvation.
Moksha salvation is gained after reaching state of enlightenment, self realization god realization. And for this we have to demolish our ego in totality. Furthermore we have to overpower five senses and mind in totality. This is a very lengthy process, takes not only decades but centuries.
Furthermore, practicing absolute celibacy for a minimum period of 12 years in continuation human beings finally succeed in full awakening of kundalini energy, the coiled Serpentine energy lying dormant in base of spine. Full awakening of kundalini leads to 100% activation of brain. 100% activation of brain means becoming an enlightened one, a bodhi, a Jina that very moment.
In the state of enlightenment, self realization Sahasra Pushpa (thousand petalled Lotus) existing in our brain behind forehead opens fully. One finally becomes a pure soul atman in the cosmic journey of life. For a pure soul atman in absence of dross impurities within the need to manifest a human body again ceases to exist. This liberated soul atman finally reaches moksha salvation, a state when it merges with God, enters kingdom of God (Vaikuntha in Hinduism).