OER are teaching and learning materials that are freely available for everyone to use. These resources are licenced under Creative Commons licences. CC licences allow material to be used for educational purposes. As a result, lecturers can freely copy, share and sometimes modify and remix a CC work without having to seek permission from the creator.
Your obligation when using OER is to attribute the creator or source of the material.
Use Google to discover Open Educational Resources:
Search Google. To filter results to OER (with Creative Commons licences) choose ‘Settings’ under the search box.
From drop-down box select ‘Advanced Search'. Scroll down to ‘usage rights’ and select ‘free to use, share or modify'.