The physician statement is key for both. And, while a statement from a psychologist will be helpful in backing up the physician's report, both OPM and Social Security require a physician (either MD or DO is fine), and it would be most convincing coming from a psychiatrist. Nurse Practitioners and Physician Assistants won't do either - it has to be a board-certified physician. With OPM, a good statement from your supervisor will help too (they require one) and you also need to assess whether or not you could do your job with accommodations. Who is prescribing your meds?