What is the ruling on one who suffers from severe obsessive-compulsive disorder (OCD) that drives him to repeat ablution (wudu') and prayer, and the opening Takbir (takbirat al-ihram), and to repeat previous prayers when the time for the next prayer begins, in addition to frequently swearing an oath to do only one thing but then breaking his oath?

The cure for obsessive thoughts (waswas (definition)) is to ignore them and not pay attention to them, seeking Allah's help and relying on Him. You should perform ablution once, say "Allahu Akbar" once, and pray once. Do not repeat ablution, "Allahu Akbar," or the prayer, no matter how much Satan whispers to you. Visiting a trustworthy doctor and seeking medical treatment is permissible. Remember that intentionally delaying a prayer beyond its time is among the major sins, so you must perform each prayer at its appointed time.