If I say something, of course it's my opinion, or I wouldn't say it, so I cannot see the point of "IMO".

The exception would be if I were speaking ironically, and it wasn't really my opinion, and in that case of course "IMO" woudl be a lie.

Perhaps it would make sense to put in a irony warning of some kind at the end in such cases, to avoid confusion. Oddly enough, I don't think there is any generally agreed way of indicating this. IDRMT for "I don't really mean that" probably wouldn't be generally understood.