The Medicare Supplement Open Enrollment Period begins on the first day of the month that you are enrolled in Medicare Part B. It lasts for 6 months. In order to join a Medicare supplement insurance plan you must be enrolled in Medicare Part A and Part B.
There is no health screening during this period, so you cannot be denied coverage if you enroll during this time. You can still apply for a supplement plan after this time, but based on your current health condition and risks, you may be denied coverage or charged a higher premium.
In other words, an individual with a pre-existing health condition who enrolls in a supplement plan during this Open Enrollment Period, will be paying the same premiums as a perfectly healthy individual.