The [Journal of Nonsmooth Analysis and Optimization](https://jnsao.episciences.org) 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](https://gcc.episciences.org) and the [Epijournal de Géométrie Algébrique](https://epiga.episciences.org).

<sub>*(Disclaimer: I am one of the Managing Editors of JNSAO.)*</sub>