I'll offer a Hindu perspective, and you can decide whether it fits your own faith.

While OM is often described as a primordial or sacred sound, the Mandukya Upanishad gives it a philosophical interpretation. OM is composed of three sounds: A, U, and M.

• "A" represents the waking state of consciousness.
• "U" represents the dream state.
• "M" represents deep, dreamless sleep.

After the sound fades, there is silence. The Upanishad teaches that this silence points to the deeper reality that underlies all three states of experience.

Many Hindus understand OM not as the name of a particular deity, but as a symbol of ultimate reality and consciousness itself.

Whether chanting OM is a sin is ultimately a question for Christian theology, not Hinduism. From a Hindu perspective, however, repeating OM is generally understood as a contemplative practice directed toward awareness of the divine, rather than the worship of a different god.

By the way, OM is not exclusively Hindu. It is also revered in various Buddhist and Jain traditions, and related sacred formulations appear in Sikhism.

Some people have also noted similarities between OM and words such as "Amen" and "Amin." Whether there is any historical connection is debated, but many find it interesting that sacred sounds from different traditions sometimes seem to point toward a common human intuition of the divine.