Do I need to wear a medical identification bracelet if my blood type is O-?

If you are prone to seizures, you should wear one, so that in case of one and nobody knows what is happening or why, the people around you & the paramedics and other professionals will have a big clue to better care for you.

The seizures, regardless of the cause, should be on an ID bracelet.  They sell little jumpsticks that will hold your entire medical history all the meds, allergies, contact information, etc.  That makes it easy for the ER staff when you are brought in not able to talk to them.  IF you needed blood, they would not take your word for it, but run a type and cross-match.  So that is not as important as the seizures.  I would worry more about getting another brain injury than bleeding.  If your seizures are not controlled, perhaps you should talk to your Neurologist about them

It would be a good idea to get a bracelet but you don't need one for your blood type. You will never get any blood other then O neg (universal donor)  if you require a transfusion without a blood type and crossmatch first - even if you tell them what blood type you have, it will still be tested.