Start with the memo written by John P. Holdren, Obama's director of the Office of Science and Technology Policy.
https://obamawhitehouse.archives.gov/sites/default/files/mic...This is a "meta-policy" of sorts, in that it directs grant-making agencies to form policies of their own. A lot was written about it at the time, most of which includes its title "Increasing Access to the Results of Federally Funded Scientific Research."
It came up again in 2020 when OSTP considered revising the rules (e.g., https://www.sciencemag.org/news/2020/05/will-trump-white-hou...)
Depending on your interests, you could then look at individual agencies' policies.
- Here's the NIH policy: https://publicaccess.nih.gov
Search for something in PubMed and there will be a "PMC Free Full Text" icon if it's available from PubMed Central. Some publishers just lower the paywall, in which case it says "free from publisher".
- An FAQ for the NSF: https://www.nsf.gov/pubs/2018/nsf18041/nsf18041.jsp
- The DoD hosts its own repository called DTIC. Heres' the policy stuff: https://discover.dtic.mil/policy-memoranda/ Open access is the last section. You can search at the top-right for DoD funded research.
- Not to be outdone on the acronym front, the DOE has a site called PAGES, which ostensibly stands for Public Access Gateway for Energy and Science: https://www.osti.gov/public-access
- Those are the biggies, but there are a ton of federal agencies covered by the policy. Here's the USDA's plan: https://www.usda.gov/sites/default/files/documents/USDA-Publ...