There are different ways to publish Open Access:
You can find a list of Gold Open Access journals and Diamond Open Access journals on the DOAJ website, filtering for Without fees if you want only Diamond. You can also look at the Sherpa services site to find out which are the policies of the journals toward Open Access.
When you are publishing a paper, you should pay attention to both copyright and licences, because a licence complements copyright, adding new specifications.
If you sign a contract in which you give all your rights to the publisher, you won't be able to publish your paper in an institutional archive such as Aisberg, or to reuse or re-publish it in any other way (in this case, you can't share your paper even with colleagues in ResearchGate or Acadamia.edu) . So be careful about which contract you sign and possibly ask to retain the right to publish your pre-print or post-print in the institutional archive. See the Sherpa services site to know which kind of contract the various publishers offer.
Best option is to use a Creative Commons licence. With this kind of licence, the author retains his rights, establishing what the users can or cannot do with his paper. There are six different Creative commons licences, as you can see here.