How does a worshiper suffering from obsessive-compulsive disorder (OCD) in prayer get rid of it, especially regarding the opening Takbir (Takbirat al-Ihram), determining the intention of the prayer (obligatory or supererogatory), reciting Al-Fatiha, and raising his voice with some of its verses?

Waswasa (whispers/obsessive thoughts) is one of the most dangerous afflictions that corrupt both one's religion and worldly life. The best cure for it is to ignore it and pay it no attention. All these obsessive thoughts are from the plots of Satan, so do not give him an opportunity to plant seeds of evil in your soul. Disregard any waswasa that is cast into your mind and give it no importance. If Satan whispers to you that you did not intend the obligatory prayer, then immediately say "Allahu Akbar" (the takbir) and tell him, through the state of your being, that you did indeed intend the obligatory prayer, for it is inconceivable that you came to pray only the Sunnah (supererogatory) prayer. Hasten to say the takbir immediately after the imam's takbir without hesitation, and recite naturally, without affectation, and do not raise your voice in recitation behind the imam. Strive to stop repeating verses or words. If Satan whispers to you that you made a mistake in recitation, pay no attention to it and continue with your worship, for not acting on this advice will cause you distress. Strive against yourself, seek Allah's help, and supplicate to Him to remove the waswasa from you.