Open access journals that provide immediate open access to all of its articles on the publisher's website
A version of the published article is placed in an institutional repository (such as Unitec's Research Bank) , central repository (such as PubMed Central), or on an open access website for free public access. OA repositories can contain preprints, postprints, or both. A preprint is any version prior to peer review and publication, usually the version submitted to a journal. A postprint is any version approved by peer review.
Sometimes it's important to distinguish two kinds of postprint: (a) those that have been peer-reviewed but not copy-edited and (b) those that have been both peer-reviewed and copy-edited. Some journals give authors permission to deposit the first but not the second kind in an OA repository. OA repositories can include preprints and postprints of journal articles, theses and dissertations, course materials, departmental databases, data files, audio and video files, institutional records, or digitized special collections from the library.
Authors need no permission for preprint archiving. When they have finished writing the preprint, they still hold copyright. If a journal refuses to consider articles that have circulated as preprints, that is an optional journal-submission policy, not a requirement of copyright law.
Journals with individual open access articles for which their authors (or their author's institution or funder) pay an OA publishing fee
Traditional subscription-based journals that provide open access or free access upon the lapse of an embargo period following the initial publication date (with the embargo length varying from a few months to two or more years). A journal subscription or an individual article purchase fee would be required to access the materials before this embargo period ends. Some delayed open-access journals also deposit their publications in open repositories when the author is bound by an (immediate or delayed) open access mandate.
There is a misconception that OA journals do not provide peer review or the same level of peer review as other publishers. Open access publishing provides a level of peer-review with the same standards and procedure and even the same reviewers as conventional journals.