Ong namo guru dev namo is a powerful mantra used a lot in Kundalini yoga. It can be broken up as:
Ong Namo – I bow to the subtle divine wisdom
Guru Dev Namo – I bow to the divine teacher within
It was taught by Yogi Bhajan to allow yourself to relate to that divine teacher within yourself.
I like to use it at the start of a meditation. Mantras have powerful energy behind them. This one is great if you feel you need to connect with your higher self. It opens your mind to look past your ego and to accept Source energy.
It can be sung or spoken. Here is a link to a musical version: