The Journal of Nonsmooth Analysis and Optimization is a "fair" (in particular diamond) open access journal that launched this year on the episciences arXiv overlay platform.

Other journals on that platform that I haven't seen mentioned yet are the journal of Groups, Complexity, Cryptology and the Epijournal de Géométrie Algébrique.

_{(Disclaimer: I am one of the Managing Editors of JNSAO.)}