How can you define mental illness behaviour or just plain odd behaviour from people that just have different ways of doing things ?

Odd behavior: is not an issue unless it interferes with your success at work, school or with interpersonal relationships. if the issue has persisted for more than 6 months you may just be dealing with a personality trait or style. Psychopathology requires that one is at risk for harm to self or others. I hope this is a useful starting point ( without delving into specifics)

Mental Illness: I understand your concern. As Individuals, we have different ways of doing things. “plain odd behavior”, not affecting functioning in life, and not causing harm to others is not mental illness. Mental illness behavior can be -acting on hallucinations, delusions or paranoia. People make judgements about what they think is odd behavior and criticize others. Therapy will be helpful to deal with them