There are a fair number of sites that index OA journals. The Directory of Open Access Journals (DOAJ) is one of the best known. Additionally, the DOAJ is searchable in many online library catalogs, including ours.
Even if you initially discover an OA journal through our OneSearch catalog, search for it in the DOAJ search by title. This will allow you to see a snapshot of information regarding the publisher's open access policies. For example, by using our Periodicals search, we can find an example open access journal, the Iberian journal of the history of economic thought.
From there, we can search for this journal in the DOAJ. It returns a result that already begins to tell us a little about this journal's policies.

Screenshot of a query result in the DOAJ. Article metadata copyrights and related rights waived via CC0 1.0 Universal (CC0) Public Domain Dedication by DOAJ.
From here, clicking on this query result brings us to a journal overview that provides more details.

"Screenshot of journal overview page in DOAJ for Iberian Journal of the History of Economic Thought" captured from the DOAJ journal overview page for this journal.
If you have a particular publisher that you are interested in, individual publishers often have search pages to help you navigate their OA journals. Then, there are entire publishing organizations that only publish OA.
If you want to investigate the OA policies of individual publishers to decide whether they are in line with your goals and/or meet funder requirements, see:
There are additional ways to locate OA journals, as through a serials indexing database like Ulrichsweb, but at this time we do not subscribe to them.