Locating and using openly licensed and public domain content is a great alternative to requesting permissions or doing a fair use analysis on copyrighted educational content. You may also consider open content if you find that the materials you want to use may not weigh in favor of fair use. The following repositories are a good starting point for your search:
For more information on Open Access and Open Educational Resources, consult the following guides:
Open Access is an umbrella term for a variety of publishing models that allow published works (typically scholarly journal articles and conference proceedings) to be made freely available online (libre and gratis). Often the open lock symbol indicates that a work is published under an OA model.
Green OA is a model that allows an author to deposit a pre- or post-print in an institutional repository (IR) such as UO Scholars' Bank. IR content is indexed by search engines like Google Scholar and discoverable to a worldwide audience.
Gold OA is a model from publisher's that requires the author(s) pay the cost of publishing and allows the Version of Record to be distributed online in a free (Gratis) manner through journal websites, library subscription databases, or online search engines.
Platinum OA is a model like Gold OA where the Version of Record can be distributed online in a free/Gratis manner, but the cost of publishing is passed along to a third party, like a sponsoring university or corporation.
Pre-print (a submitted Manuscript that is under review): This is the version of the article you submitted to the journal i.e. prior to peer review.
Post-print (aka the author's Accepted Manuscript): This is the version of the article accepted for publication i.e. after peer review. This is your copy, not the final publisher version.
Version of Record: The final, published version of your article, often in PDF format with the journal's style and branding.
Libre vs. Gratis - Gratis OA is a term that means for free (removal of price barriers), while Libre OA means free (removal of price barriers) and free of most copyright restrictions (removal of permissions barriers).