I've noticed that many students choose not to chant the three 'OM's with me at the beginning of a session and that is totally up to them, no issue with it, but! I believe that people may be more inclined to do so if they knew a little more about it and I feel it's my duty to share that as a Yoga Teacher.
I really would suggest you forget your 'ego' or feeling 'weird' and just try it. If nothing else and you don't believe in the spiritual energies, it's paying respect to the practice.
So why do we 'OM'?
To clarify, it would be much more accurate to write it as AUM.
A - Represents physical strength - Vibration are in the stomach