The decision belongs to the journal owner or publisher, not the editors.
But editors have some leverage. They can get the ear of the publisher more easily than outside advocates can, and the publisher usually want to keep them happy. But this leverage rarely suffices.
Sometimes when editors try to persuade a publisher to flip a journal to OA, and fail, they resign en masse and launch a new OA journal in the same niche. This is called a declaration of independence. See the crowd-sourced list of them from the Open Access Directory.