My dad has a mental illness and I don't know how to deal with it. He doesn't even know he has one. He is severely paranoid, obsessing over certain topics, etc?

A wonderful licensed MFT once advised me to always "check the facts and evidence" of paranoid thoughts in mentally ill patients. To answer this question, it might be beneficial to ask father when he is acting severely paranoid, obsessing over certain topics, etc. to ask who, what, when, how, and why questions to help understand what time frame he might be re-living (of course if you have the patience). It doesn't have to be thorough but maybe ask "what makes you say that?" to prompt further inquiry of what he may be thinking. Also, prompting and redirecting him to the 'here and now' can help redirect him to the present and help him get 'unstuck' in his line of thinking.