There are two main paths to Open Access for scientific publications:

1. Green Road, DEPOSIT or SELF-ARCHIVING
   The author deposits a version of the published article in open online archives (institutional or disciplinary). Some publishers require that the full-text of the document is only opened after expiry of an embargo period (a variable term depending on publishing policies). The bibliographic metadata (title, author, abstract, keywords, DOI etc.) that accompanies the publication deposited in an open archive is always visible to everyone, including during the embargo period. 70% of publishers permit deposits. Self-archiving is always cost-free for  the author, and makes it possible to meet the OA requests of the funding bodies.
2. Gold road, OPEN ACCESS PUBLICATION.                                                                                                                                                                                          The author publishes the accepted article directly in Open Access journals, in an open way that makes it accessible to everyone, at no cost. Publication costs are thus shifted from the reader to the author: the most frequent economic model provides for a one-off payment (APC - Article Processing Charges) by the authors.

FAQ

How do I find out which version of a work can be deposited?

You can search for the publishing policy of the journal to which you submitted your article, entering the journal’s title in the SHERPA ROMEO database. The database underwent a complete overhaul in 2020, with new graphics and a new system of icons which illustrate  (according to the version of the article you wish to deposit), the conditions required by the publisher (embargo, payment, Creative Commons Licenses to be associated, etc.). See how it works in this short tutorial (Video by U.P. Open Access of the University of Turin, CC-BY 3.0.) Not all publishers, especially in Italy, are registered on the Sherpa Romeo database. If not, you can search for the self-archiving policies directly on the publisher's website.

Which versions of the article can be deposited?
PRE-PRINT or SUBMITTED MANUSCRIPT: an article completed by the author, but not yet peer-reviewed or edited by the journal;
POST-PRINT or ACCEPTED MANUSCRIPT: an article ready for publication, integrated with reviewers' comments and identical in all respects to the published version except for the editorial layout;
EDITORIAL PDF or PUBLISHED VERSION: the same version of the article as the one that will be or has been published in the journal, including layout and graphics; it is known as the editorial version, regardless of whether it has already been published or not.

In a nutshell, which editorial conditions must be verified before depositing in a repository?
Which version of the article can be deposited and when it can be made publicly available.

Does depositing in Open Access make plagiarism easier?
No, quite the opposite: by depositing in an open archive, you obtain certification of intellectual paternity associated with a definite date.

Why is it important to deposit?
To make the full text of the research results visible, accessible, searchable and usable by any potential user that has Internet access 

Can depositing be in breach of the agreement with the publisher?
No, because only the version authorised by the publisher is deposited, in line with conditions that can be found on Sherpa Romeo, or on the publisher's website.

What are OA journals?
The Directory of Open Access Journals (DOAJ) includes free journals for the reader and the author, edited by research groups or scientific societies that guarantee selected content. Many of these journals use the Open Journal Systems (OJS) as publishing software, which tracks peer reviews. Many Open Access journals publish the reviewers’ scrutiny together with the article, to ensure transparency, or allow an Open Peer Review or a post Peer Review, enabling experts’ comments.

Do you have to pay to publish OA?
Native OA journals do not require publication fees in most cases (see DOAJ). However, some publishers, both Open Access-only and traditional publishers offering an Open option, require the payment of Article Processing Charges (APC). If the publication involves a cost, the publication fees requested by the publishers may be charged to the author or their institution. In some cases these costs can be covered by a research-funding body. An example of such a body is the European Commission, which considers PCAs as eligible costs (see pages on H2020 and HEUROPE).

If I choose the Gold road, must I necessarily upload to an institutional Repository?
Yes, both to ensure long-term retention (which is not guaranteed either by the publisher's platform or by commercial services such as ResearchGate) and for research evaluation reasons.

Are all Open Access publishers predatory publishers?
The phenomenon of predatory publishers is not connected to Open Access. Although there are publishers who use OA to publish journals of dubious quality, labelling the phenomenon as OA publishing is reductive and misleading. The purpose of predatory publishing is certainly not to disseminate the results of scientific research but to profit from poor quality services, exploiting the logic of open access as an ethical way of conducting research.  

How to recognize a predatory publisher?

Predatory publishers are not immediately identifiable, as they create entirely realistic websites and may claim that they have (bogus) bibliometric indicators or that they are indexed in known bibliographic databases. Useful tools for ascertaining whether a publisher is reliable include the directories of DOAJ (see previous FAQ) and OASPA Open Access directories and publishers, which only list publishers who have achieved high scientific and ethical quality standards. The Think-Check-Submit website provides researchers with a checklist to examine a journal or publisher, and ascertain their reliability.

Some tips:

• Don't trust unsolicited emails;
• View recent issues of the journal;
• Make sure the journal has a registered ISSN;
• Check the journal's leadership and affiliations: Are you familiar with the names of the editorial board members? Is the journal listed in the DOAJ? 
• Are manuscript acceptance times too short?
• Are publication costs transparent?