OER are educational resources that can be openly used, adapted and republished – for any purpose.
Creative Commons licences, especially CC0, CC-BY and CC-BY-SA licences, allow this type of subsequent use.
The CC licence represents permission for the free use of copyright protected works under the conditions of the respective licence. It is not required to obtain separate permission from the authors or to conclude an individual agreement.
You can also find answers to many of the questions you have about OER in the “Creative Commons licences” section of this FAQ list.