Is uncovering a part of the body in front of a strange man online considered apostasy (kufr)? And is it obligatory to perform ghusl (ritual bathing) to embrace Islam in this case, especially with the presence of obsessive-compulsive disorder?

What you did is a sin that necessitates repentance. If you repent sincerely, feel remorse, and resolve not to return to it, then Allah will accept your repentance, and you will become as if you never sinned, for the Prophet, peace and blessings be upon him, said: "The one who repents from sin is like one who has no sin." This is not disbelief, and what you mentioned has no basis in truth. You must struggle against these whisperings (waswas) and turn away from them, for succumbing to them leads to great evil.