If the school you named as your first choice for Open Enrollment does not have a waitlist, then you are in. That's confirmed.
My daughter and her friend named Chief Sealth as their first choice for assignment. Chief Sealth doesn't have a waitlist, so they know for sure that they will be assigned there even though they don't yet have their assignment letters.
You can check your school of choice from this web page to see if the school has a waitlist.