How does a person afflicted with obsessive-compulsive disorder (waswas) distinguish between ghusl from janabah (major ritual impurity) and any other ghusl, and how does he specifically intend for ghusl from janabah? Is doubt during wudu (ablution) significant, or should it be disregarded unless one hears a sound or perceives a smell?

The cure for waswasa (whispers of Satan/obsessive doubts) is to ignore it and not pay attention to it. If you succumb to it, you will fall into great evil. The matter of intention (niyyah) is easier than you imagine. Intention is the resolve to perform the intended action, and its place is the heart. So, you should intend with your heart what you wish to do, whether it's ghusl (ritual bath) or anything else. There is no need to utter it verbally, as that only increases waswasa. Intention follows knowledge; whoever intends to do something must have the intention for it. If you wish to perform ghusl for janabah (major ritual impurity), bring to your heart the intention of removing the impurity or making prayer permissible, and do not pay attention to the whispers of Satan. Similarly, do so with doubt concerning tahara (purity) or after it; turn away from doubt unless you gain certainty that your tahara has been invalidated. There is no difference between doubting the invalidation of tahara after performing wudu' (ablution) or during it. The default is the absence of hadath (impurity), and it is only removed by certainty of equal measure. Completely close the door to waswasa, and do not allow Satan to corrupt your religion and your worldly life.