How can one definitively get rid of obsessive-compulsive disorder (OCD)? Is it permissible to perform ghusl for janabah by doing wudu and then spreading water over the entire body? Is ghusl obligatory when one feels nocturnal emission without any trace of semen, but with very slight wetness on the clothes?

Treatment for waswas (whispers/obsessive thoughts) is to turn away from them, ignore them, and persist in striving against them.

As for ghusl al-janabah (ritual bath due to major impurity), it is valid by covering the entire body with water in any manner, and a strong presumption is sufficient for this. Ghusl is not obligatory unless there is absolute certainty of the emission of semen, which necessitates the bath. If no semen is seen after a wet dream, or if there is doubt about its emission, then ghusl is not obligatory. However, if semen is emitted, even in a small amount, then ghusl is obligatory.