What is the effective treatment for Obsessive-Compulsive Disorder (OCD) related to pessimism and optimism, and does God hold one accountable for the compulsive actions resulting from it?

As long as you know the absurdity of these thoughts, you must turn away from them and strive against your soul to repel them. If these whispers overwhelm you to the point that you cannot دفعاً (repel) them, then that will not harm you—by Allah's permission—but do not surrender to them; rather, strive against them and repel them.

As for believing these thoughts out of compulsion, without your choice, we hope there is no sin upon you. And know that sins have an effect on making matters difficult. Pessimism and ill omens are practices of the people of Jahiliyyah (pre-Islamic ignorance).

As for good omens (al-fa'l), the Prophet, peace and blessings be upon him, was pleased by them, and they are the good word. Accustom yourself not to transgress the limits of the Shari'ah, and seek help through supplication and the established adhkar (remembrances).