What is the difference between the doubt of one afflicted by obsessive-compulsive disorder and the saying of the Prophet, peace be upon him, "without doubt about them"? And is it permissible for someone afflicted with obsessive-compulsive disorder to marry a Muslim woman?

Doubt differs from waswasah (whispers/obsessive thoughts). Waswasah is an unchosen assault on the heart, and disliking it indicates pure faith. As for doubt, it is the opposite of certainty and is a state of hesitation. A servant is not a believer if they fall into doubt; rather, they must be certain and resolute in "there is no god but Allah, Muhammad is the Messenger of Allah," as Allah Almighty says: "Indeed, the believers are those who have believed in Allah and His Messenger and then have not doubted," and as the Prophet, peace and blessings be upon him, said: "No servant meets Allah with them [the two testifications of faith], without doubting them, except that he will enter Paradise." The repentance of one who doubts is by uttering the two testifications of faith with certainty. One might mistake waswasah for doubt, but their dislike of it is proof of their faith. However, following waswasah and the unsettling of certainty raises concerns about the faith of its owner. Obsessive-compulsive disorder is not a defect in marriage.