There are several Open Access publication models available:

Green Open Access

To make an article immediately available in Open Access, which is expected to be published in a subscription-based journal without editorial charges or article processing charges (APC). It is possible to deposit an open version of the work through self-archiving (or Green OA). This strategy involves uploading the work, in the version allowed by the publisher, to an institutional or disciplinary thematic Open Access repository, with no additional costs for the author. Versions allowed for Green OA deposit, which vary depending on publisher policies, include the pre-print or submitted version (pre-peer review), the post-print or accepted version (post-peer review, without the final editorial formatting, also known as Author's Accepted Manuscript), and, to a lesser extent, the publisher's version (Version of Record - VoR).

Publishers often require an embargo period (typically for the accepted manuscript and publisher's version) before the contribution can be made open; the embargo end date should be specified during the deposit of the document in the institutional repository and the embargo period should be calculated from the date of online publication on the publisher's site.

The Green OA contribution can be accompanied by an open reuse license, chosen by the authors or defined by the publisher’s policy.

The tool to check publisher policies regarding the versions that can be deposited and the methods is Open Policy Finder, a platform that collects information on publisher and journal policies.

Hybrid Open Access

Publishing in a subscription-based journal, paying APC to make the article immediately available for free on the publisher’s site, with an added open reuse license (usually CC – Creative Commons). However, the Hybrid OA model involves the phenomenon of increased costs for reading and publishing, known as double dipping, which is addressed by research and academic institutions through transformative publish & read agreements.

Gold Open Access

Publishing in a fully Open Access journal, and, after paying APC, the published contributions are immediately made available by the publisher on their site. Generally, copyright is retained by the authors, with a license allowing various levels of reuse (typically CC – Creative Commons). Discounts on APC payments may be available to authors through specific transformative publish & read agreements.

Platinum o Diamond Open Access

Publishing in a fully Open Access journal where published contributions are immediately available on the publisher’s site without any APC charges.

Copyright can be retained by the author, and open licenses allow sharing and reuse (generally via CC – Creative Commons licenses). 

The Directory of Open Access Journals (DOAJ) helps to easily identify non-profit Diamond academic journals.