Open access is a process that makes journal articles freely available to the general public. Open access pushes back against "the traditional model of publishing whereby authors sign their scholarly work over to publishers, who in turn charge academic institutions and the general public large sums of money for access to this information. In general, the open access movement supports the idea that research--particularly research funded by the public--should be available to the public at no charge" (Washington State University).
There are four types of open access models:
NIH Public Access Policy
The NIH Public Access Policy ensures the public has access to the published results of NIH-funded research, and requires scientists to submit the final peer-reviewed journal manuscripts that arise from NIH funds to PubMed Central (PMC) upon acceptance for publication. The policy requires that these papers become accessible via PMC no later than 12 months after publication.
Learn how to comply with the policy here.
Plan S
Plan S, launched in 2018, is "an initiative for Open Access publishing... supported by cOAlition S, an international consortium of research funding and performing organisations. Plan S requires that, from 2021, scientific publications that result from research funded by public grants must be published in compliant Open Access journals or platforms" (Source). Because a significant percent of American researchers collaborate with colleagues outside the U.S., any works arising from their research will have to be published in Plan S-compliant journals. A growing number of national funding agencies and charitable organizations worldwide are committed to funding and supporting the principles of Plan S. See a list of the participating funding agencies here.