Re: Obsessive Gum/Mouth/Lip Chewing
If you are concerned about your health in general the first thing you should probably try to do is give up smoking and be careful about drinking, especially if you are still young.
Many healthy people have behaviors like chewing on lips or finger nails or tapping their fingers, etc. If doing so is not causing your pain or discomfort or getting in the way of your life you probably don't need to worry about it although you could always check with your doctor. If these behaviors are causing physical damage and/or if you have tried to stop doing them but find that you can't then you may need to follow up as it could be a problem.
As far as cancer goes, your smoking and drinking are the risk factors, I'm no doctor but I don't think I've ever heard of cancer being related to chewing on your lip.
Based on your description I wouldn't think its OCD related, though that isn't a guarantee, and it may be related to some other condition that you can get help with if its a concern for you. OCD presents itself through obsessions and compulsions. The obsessions are troublesome, worrying, unwanted thoughts that you can't seem to make go away and the compulsions are behaviors that you feel compelled to engage in to hopefully (and temporarily) alleviate your anxiety. It could be possible that your behavior is related to OCD if you feel compelled to do it because of some worry and find that engaging in the chewing helps alleviate that worry, but you should really see a qualified mental health professional to make a determination. In your case it seems like the flow is reversed though (which makes me doubt the OCD), you've identified your behavior and are concerned it may cause harm, rather than the reverse of engaging in a behavior to AVOID harm.
My laymans guess? Its just a bad habit you've picked up over the years, but it never hurts to talk with your doctor if you want some professional advice.