OneSearch indexes open access content from numerous repositories like the Directory of Open Access Journals. See Finding an article in OneSearch.
Check Google or Google Scholar to see if the article is available for free.
Tip: In Google Scholar, it can be tricky to differentiate between open access content and articles that require a library subscription for access. One solution is to use a browser plugin like EndNote Click, a free time-saving tool compatible with Google Scholar, PubMed, Web of Science, and numerous other academic sites. It quickly retrieves PDFs from open access sources and library journal subscriptions.
Other online tools can help you find free and legal open access versions, including preprints deposited into repositories by authors.