The Open Enrollment Period is a 6 month period when you can buy any Medicare Supplement policy without being denied. The 6 month period begins the first month that you are signed up for Medicare Part B. During this time you cannot be denied a Medicare Supplement policy or be charged more due to health history.
Guaranteed Issue is defined as the rights that you may have in certain special situations when insurance companies are required to sell you a Medicare Supplement policy. An example of this is the Special Election Period that occurs when a person comes off of group coverage or if you move out of your Medicare Advantage Service area or if your Medicare Advantage carrier drops coverage.