"the page opposite the editorial page"
But I'm not sure this is its "full name." I think the full name of this section of the paper IS "Op-Ed."
It is a space reserved for the publication of the public's opinions, especially (but not only) as a way of giving readers the opportunity to publish an opinion in opposition to the editorial opinion of the newspaper.
The name comes from the set-up of the newspaper: Somewhere near the middle of the newspaper is the Editorial Page on the left hand side, and the Op Ed page on the right -- the page opposite the Editorial page. (If there isn't a lot of either one, sometimes the Editorial pieces and the Op Ed letters and articles are just put on the same page.)
Op-ed - Wikipedia, the free encyclopedia
Op Ed submissions are sometimes the first thing a new author ever gets in print. The publication of several good free-lance Op Ed articles can help establish a new author. The articles don't especially have to be in contradiction to the Editorial position of the newspaper, or even on any of the same topics.