If you've investigated publishing in an open access (OA) journal, you've likely heard of predatory journals or publishers. These publishers, while certainly not exclusive to OA journals, tend to target authors seeking to publish in OA journals or may contact authors directly asking for manuscripts. They exist primarily to gain profit.
Frequently, these publishers do not initially mention an author's fee, but may later ask for a publication fee under the guise of operating as a quality journal.
Membership in the Directory of Open Access Journals or the Open Access Scholarly Publishers Association is a good indicator that a journal is not predatory. You can check these sites to help you determine that the journal you are interested is legitimate.