There are many ways to access content that has been made available either through the Green or Gold open access routes.
For content published in open access, you can go directly to the journal or publisher’s website, for example PLoS ONE offers a search function for all its published content.
The Directory of Open Access Journals provides article level search for over 6700 open access journals in all fields.
Aggregators like BASE and Google Scholar provide access to a mix of green and gold OA content from publishers, repositories, and websites.
You can often find the latest research in different fields by searching a subject repository like ArXiv, PubMed Central, SocArXiv, and many other subject repositories, where researchers will post their pre-prints and finished articles.
Tools like oaDOI, DOAI, and the Open Access Button are designed to help you find a free version of paywalled articles. These work by using the DOI, a unique identifier for the article, or the URL of the article, to locate a version posted in subject or institutional repositories.