Is Allah's erasure of sins for every disease applicable for mental diseases like OCD too?

OK, the principle in the hadith is general. The Prophet, peace be upon him, said: 'No Muslim is afflicted with any harm, even if it is a thorn prick, but that Allah will expiate his sins because of it.' The word used is 'harm' or 'affliction.' This includes physical and mental afflictions. So yes, OCD, anxiety, depression—these are all afflictions. If the person bears them patiently, seeking reward from Allah, then they become a means of expiation of sins and elevation of ranks. This is a mercy from Allah.