The biggest difference between open access materials and public domain materials is that open access materials are protected by copyright, but public domain materials are not.
If I create a work, like a cartoon, and I want to keep some rights to myself but then give users some rights to use it, then I attach an Open Access licence to the work, like a Creative Commons licence.
What rights am I talking about? I’m talking about the exclusive rights that belong to copyright owners in Australia: The rights to copy, communicate, publish, perform, and adapt the work.
If I attach a Creative Commons licence, now that work is publicly accessible for free, and users can use it within the limits of the licence.
However, if I create a work, like a cartoon, and I don’t really need to keep any rights (as in, I don’t really need to control the copying or communication of the work,) then I can release it into the public domain. That basically means that I’m giving up my copyright.
Public domain is sometimes indicated by the Creative Commons licence CC0, which means zero rights have been reserved by the creator.
The phrase public domain means that there’s no more copyright in the work, so works for which copyright has expired are also in the public domain.
Once copyright in a work has expired, the moral rights have expired as well, and there’s no requirement to attribute the work to its creator. However, citing sources is always good practice.