Open Access Publishing: Models, Mandates, and Why It Matters

Open access (OA) publishing makes peer-reviewed research articles, and increasingly books, data, and other scholarly outputs, freely available online for anyone to read, download, and reuse — without a subscription, paywall, or per-article purchase. It has moved from a fringe advocacy position in the early 2000s to the default expectation of most major research funders, and understanding how it actually works — who pays, which model applies, and what a given funder mandate requires — is now a routine part of publishing a paper.

This guide is the foundational overview: what open access means operationally, why funders and institutions have moved toward requiring it, the four commonly-used OA models, how licensing works, and where to find CASRAI’s more detailed coverage of each funder mandate and cost mechanism.

What “open access” means operationally

CASRAI’s open access dictionary entry defines it as making peer-reviewed scholarly content freely available via the internet, with users permitted to read, download, copy, distribute, print, search, link to, and text-mine the full text without financial, legal, or technical barriers beyond internet access itself — a definition tracing to the foundational statements of the open access movement (the Budapest, Bethesda, and Berlin declarations of 2001–2003). The only right an author retains under this definition is control over the integrity of their work and the right to be properly credited and cited.

Two things this definition deliberately excludes: OA is not the same as “free to read” alone (a publisher can make an article temporarily free without granting reuse rights), and it is not the same as unreviewed self-publication — OA describes an access and licensing model layered on top of, not instead of, ordinary peer review.

Why open access publishing matters

• Funder accountability. Research funded by public money or charitable endowment produces findings the funder considers a public good; nearly every major funder mandate discussed below is grounded in the argument that the output of publicly-funded research should not sit behind a second, private paywall.
• Access equity. Subscription paywalls exclude researchers at under-resourced institutions, clinicians and practitioners outside academia, policymakers, journalists, and the general public — audiences with a legitimate need to read primary research who have no institutional subscription.
• Machine and downstream reuse. An open license (see below) permits text-mining, dataset construction, and AI-assisted literature synthesis at scale in ways a closed-access, all-rights-reserved article does not.
• Alignment with the broader open science movement. UNESCO’s 2021 Recommendation on Open Science frames open access to publications as one pillar of a wider shift toward open data, open methodology, and open evaluation — CASRAI’s own research data management and research-integrity content covers the adjacent pillars.

The four open access models: gold, green, diamond, hybrid

OA is delivered through several structurally different routes, distinguished by who pays and where the free copy lives. This guide gives the short version; for the full breakdown — APC ranges, embargo rules, licensing norms, and a funder-by-funder compliance snapshot — see CASRAI’s dedicated comparison, Gold vs Green vs Diamond vs Hybrid Open Access.

• Gold open access — the publisher’s version of record is free at the publisher’s own site from the moment of publication, typically funded by an article processing charge (APC) paid by the author, funder, or institution.
• Green open access (self-archiving) — the author deposits a manuscript version, usually the peer-reviewed author accepted manuscript, in an institutional or subject repository, separate from the publisher’s site, at no publishing cost.
• Diamond open access (sometimes “platinum”) — free to both authors and readers; costs are absorbed by a society, university, library, or consortium rather than charged per article. Diamond titles are the largest single category of journal in the Directory of Open Access Journals by count, though they publish a smaller share of total article volume than the large commercial gold and hybrid venues.
• Hybrid open access — a subscription journal that also sells open access on individual articles via an APC, while the rest of the issue stays paywalled. It is the most funder-restricted of the four models because of the “double dipping” criticism (the publisher can collect both subscription and APC revenue for the same journal) — several major funder coalitions now only accept hybrid publication through a registered Transformative Agreement.

Licensing: why the license matters as much as the price

Free-to-read is not the same as freely reusable. Most OA policy frameworks specify a license, most commonly a Creative Commons license, on top of the access requirement:

• CC BY (attribution only) is the license most funder mandates require or prefer, including Plan S / cOAlition S, which requires CC BY by default for funded outputs, with CC BY-SA or a public-domain CC0 dedication as acceptable alternatives and CC BY-ND permitted only by exception.
• Rights retention strategies, adopted by cOAlition S members and others, have authors apply a CC BY license to their own accepted manuscript at submission, independent of whatever license the publisher’s own version carries — this is what allows a green deposit to be immediately, openly reusable even in a subscription journal.
• A green deposit made only under a publisher’s standard self-archiving permission, with no explicit open license attached, is technically OA-adjacent but not always “open” in the full reuse sense — a distinction worth checking before assuming a repository copy satisfies a funder’s licensing requirement, not just its access requirement.

For research data specifically (as distinct from the article itself), see CASRAI’s Creative Commons licenses for research data guide.

Funder and institutional open access mandates

Almost every major research funder now requires some form of open access as a condition of funding. Policies differ in which OA route satisfies them, how long an embargo is permitted (if at all), and which license is required. CASRAI covers several of the major mandates in dedicated guides:

• Plan S / cOAlition S — the coalition of (mostly European and international) funders requiring immediate OA with a CC BY license via gold, diamond, or a zero-embargo, rights-retained green deposit; hybrid is excluded except through a registered Transformative Agreement.
• NIH Public Access Policy (US) — requires deposit of the peer-reviewed author accepted manuscript in PubMed Central; the 2024 policy revision removed the historical embargo entirely for manuscripts accepted from July 1, 2025 onward. See CASRAI’s NIH Open Access Publishing Fee Limits guide for the funding-cost side of NIH compliance.
• NSF Public Access Plan 2.0 (US) — the National Science Foundation’s implementation of the OSTP “Nelson Memo” directive for zero-embargo public access; deposits are made via NSF-PAR.
• UKRI open access policy (UK) — in force for journal articles submitted from April 2022, requiring either an immediately-OA version of record under a permitted license or a zero-embargo green deposit.
• Horizon Europe (Article 39) (EU) — requires immediate open access with no embargo for peer-reviewed publications from Horizon Europe-funded research. See CASRAI’s Horizon Europe Article 39 (Open Access) entry.
• Wellcome Trust — see CASRAI’s Wellcome Trust Open Access Policy guide for current requirements.
• Canada’s Tri-Agency (CIHR, NSERC, SSHRC) — requires peer-reviewed outputs be freely accessible within 12 months via either immediate/delayed-OA publication or repository archiving. See CASRAI’s Tri-Agency Open Access Policy on Publications guide.

An institution or funder’s mandate is enforced independently of which OA color a researcher chooses — the compliance question is always “does this specific route, embargo, and license satisfy this specific policy,” not “did I publish open access in general.” A researcher publishing under multiple funders (common on collaborative grants) may need to satisfy the strictest of several overlapping mandates simultaneously.

What open access publishing costs

Cost varies enormously by route. Diamond and most green deposits carry no direct publishing fee. Gold and hybrid routes typically involve an APC, which can range from a few hundred dollars at a small society journal to five figures at a high-profile commercial title; many publishers offer waivers for authors in lower-income countries, and many funders and institutions cover APCs directly through block grants or transformative “Read & Publish” agreements. See CASRAI’s Article Processing Charges (APCs) for Open Access guide for the full cost breakdown, and the APC reimbursement (funder) entry for how funder-side reimbursement typically works.

Open access journals vs. subscription journals

An “open access journal” is a journal whose entire output is open (gold or diamond by default, not per-article like a hybrid title). The Directory of Open Access Journals (DOAJ) is the standard reference for verifying a journal is genuinely, legitimately open access rather than a subscription title falsely claiming OA status, or worse, a predatory journal exploiting the OA/APC model without providing real editorial review. Checking DOAJ inclusion (or the equivalent for books/monographs) before submitting or before treating a publication as compliant with a funder mandate is standard due diligence for a research office.

How researchers and research offices typically comply

1. Identify every applicable funder or institutional OA mandate for the paper (a collaborative, multi-funder paper may have more than one).
2. Check the target journal’s own policy — is it gold, green-permitting, diamond, or hybrid, and does it match what the strictest applicable mandate requires?
3. Confirm the license the mandate requires (commonly CC BY) is actually available through that route, not just that the content will be free to read.
4. If depositing a green copy, deposit the correct version (usually the author accepted manuscript, not the publisher PDF) within the mandate’s required window — increasingly zero-embargo for major funders.
5. Budget for an APC where one applies, and check institutional agreements, funder block grants, or transformative Read & Publish deals before assuming the author must pay out of pocket.

Frequently asked questions

What is open access publishing, in one sentence?

It is making peer-reviewed research freely available online, with permission for readers to read, download, and reuse it, generally under an open license, without a subscription or paywall.

What is open access (publishing) as opposed to just “free to read”?

Genuine open access, as CASRAI and the foundational open access declarations define it, includes reuse rights (an open license), not just no-cost reading — a publisher making an article temporarily free with no reuse license is not the same as open access.

What are open access journals?

Journals whose full content is open by default — gold or diamond OA, verifiable via the DOAJ — as opposed to a subscription journal or a hybrid journal that only opens individual paid articles.

Is open access publishing free for authors?

Not always. Diamond OA and most green self-archiving carry no author-facing fee; gold and hybrid routes typically involve an APC, though waivers, institutional agreements, and funder reimbursement frequently cover it.

Do all research funders require open access?

Most major public and charitable funders now do, though the specific route, embargo, and license requirements differ by funder — see the funder mandate section above.

What’s the difference between open access and preprints?

A preprint is an unreviewed manuscript version shared before or during peer review; open access describes the access and licensing terms of the final, peer-reviewed publication (or an accepted manuscript version of it). The two are related but distinct — a preprint server posting is not itself what a funder’s OA mandate requires unless the mandate specifically permits it.