You may have heard that a number of countries in Europe recently signed Plan S - so I'm here to help you navigate what this means, and how this may impact your research.
Over the past few years, more and more researchers have criticized the traditional model of publishing, where the author transfers copyright of his/her work to a publisher, who will then charge subscription fees to libraries or individual users so that they can access the contents. One of the largest for-profit publishers, Elsevier (annual revenue for 2017 was 2.48 billion pounds), is often the target of academic protests. Negotiations between Elsevier and the University of California system aren't going to well. The largest boycot to Elsevier is "The Cost of Knowledge". I personally stopped submitted my work to and reviewing for Elsevier journals after receiving take-down notices for PDFs privately stored on my ResearchGate for sending them in private messages to colleagues - which, in my opinion, is the same as sending an email to a colleague and not a copyright infringement.
Certainly, Plan S is not without criticism, and the most notable is this open letter from researchers. My main point of criticism is that it's not very clear what is expected from researchers now - this may be because most of the time I'm outside of Ecuador and have simply missed out on the requirements, but I am missing some sort of handbook for researchers on how to follow Plan S now. For "slow" journals in my field, it takes up to 3 years to go from submission to publication - so that could be a hurdle in starting to publish open access on January 1st 2020.
Plan S means that from January 1st 2020, all scholarly publications resulting from public research funding must be published in Open Access journals or on Open Access platforms. So far, 12 countries are supporting Plan S: Austria, Finland, the Netherlands, France, Ireland, Italy, Luxembourg, Norway, Poland, Slovenia, Sweden, the UK and the Gates foundation from the USA. China is planning to join soon, it seems. See here for official communication of Science Europe.
Note that "for books and monographs, the timeline may be extended beyond 2020".
Research funded by NWO, ERC, and EU Horizon 2020 falls under Plan S. Any work from other funding bodies does not.
However, in addition to the efforts of NWO, many universities have set targets for reaching a certain % of all publications as Open Access publications. For TU Delft, that target for 2018 was 60%, and will be higher for 2019.
As always, you should make sure that your work fits within the scope of a journal - so read the aims and scope of the journal, and check if they have article collections or special issues that may be particularly of your interest. If you don't know where to start, check recent papers in a journal of your interest, or check where the papers you most recently read and cited are published. You can also check the editorial board of the journal. In case of doubt, write the editor with your abstract to see if the journal would be interested in your work. Check if the journal is not a predatory journal.
A main feature of Open Access publishing is that the authors have to pay for the cost of publishing through an APC (article processing charge) - a cost that otherwise would be covered by the subscription fees. According to Plan S, "publication fees should be covered by the funders or universities, not individual researchers", and "such publication fees should be standardized and capped". Also note that Hybrid open-access solutions (such as an Elsevier journal asking you to pay an APC to make your article open access) are not supported by Plan S. You will find that some countries, such as the Netherlands, have agreements with publishers to cover the Open Access costs, and that some universities (for example, TU Delft) have agreements with Open Access publishers so that the university is charged directly and not the authors. If the journal of your interest is not covered by such an agreement, you can either ask the publisher for a waiver, or see if there are any additional funds at your university (such as an Open Access fund) to cover such costs.
Only some OA journals, that typically have a university sponsoring them, such as ACI Avances en Ciencias en Ingenierias of which I am the editor in chief (shameless self promotion) and which is funded by Universidad San Francisco de Quito do not charge an APC.
Since most OA journals are rather new, many of them are working on getting indexed in search engines such as Web of Science / ISI and Scopus. You may need to check where the journal of your interest is indexed to see if there may be a limitation there (for exmaple, if your institution only counts your papers that are published in Scopus-index journals).
An important aspect of Plan S is that authors should remain the copyright of their work, so when research should be published under plan S, you should check if the journal you plan to publish in requires a copyright transfer to them, or if you keep your copyright (open licensing model, creative commons). Check for the CC license on recent papers from the journal of your interest.