Yes you do, unless there is some legitimate medical reason for your inability to attend, in which case you may be able to delay your appointment. OWCP has every right to scheduled medical exams for you at “reasonable” intervals.
You may not bring family or friends into an OWCP examination. You may bring a physician to a second opinion (“SECOP”) examination arranged by or for OWCP.
This assumes the exam is a SECOP and NOT an “independent” or “impartial” medical exam scheduled to resolve a medical “conflict”. In that case, you are required to attend unaccompanied. If this is a SECOP exam, and you have the resources to bring another physician and wish to do so - go ahead! Tell the CE RIGHT NOW that you wish to do that. The examination may need to be rescheduled. If so, do that immediately. It is your right to have a physician attend your SECOP examination but do not delay in informing OWCP that you wish to exercise this option.