What does Islam say about OCD?
Nothing. But, it does say some things about mental illnesses in general:
"The pen has been lifted from three; for the sleeping person until he awakens, for the boy until he becomes a young man and for the mentally insane until he regains sanity." (Tirmidhi)
So, actions coming directly from mental illnesses are forgiven.
Are they accountable for sins and shortcomings that come from the inability to understand things?
No, if their 'sins and shortcomings' come directly because of the mental illness, and they can absolutely not control themselves.
Is OCD punishment for the sins?
Illnesses happen to all sorts of people (good and bad). They are not necessarily punishment for sin. They are, though, tests and trials from Allah.
Is OCD magic, bad eye?
Not necessarily. Illnesses are most probably natural. Black magic in this day and age is rare (at least in my knowledge). But, Satan is in the business of putting evil thoughts in people's minds:
"Satan circulates in the human being as blood circulates in the body, and I was afraid lest Satan might insert an evil thought in your minds." (Bukhari)
Do I have kufr, major minor or no sin at all?
Depends on if it is intentional and if you are trying your best to avoid it.
If you deliberately bring bad thoughts or feelings toward Allah do you have kufr, major, minor or no sin at all?
Deliberately (intentionally) thinking bad thoughts about Allah is a big sin. But passing thoughts that are gone in an instant and you seek forgiveness for them are forgiven:
"Allah has forgiven my followers the evil thoughts that occur to their minds, as long as such thoughts are not put into action or uttered." (Bukhari)
So, if you try your best not to linger on these thoughts and do your best to avoid situations that cause them and seek forgiveness for them, they are forgiven. I also suggest good deeds as a counter balance to help avoiding sins (ex. extra prayers).