If it is on the Internet, it is free to use, right?
Not exactly. Just because content is on the Internet does not mean it is free to be used in any situation. All original content, regardless of access, may have copyright protection, even if it is not declared.
When in doubt, treat everything as under copyright, until proven otherwise.
Open Access refers to removing barriers such as "paywalls". Open Access initiatives seek to make research articles and other works easy to find and read, for free. It does not address copyright, but rather the methods of funding and accessing the research or other works.
Open Educational Resources are works that copyright owners have "opened" by adding a Creative Commons or other License that removes some copyright restrictions. The ideal is to allow others to "retain, reuse, revise, remix, and redistribute" without needing to ask for permission, as long as the work is attributed to the copyright owner, giving credit where it is due.
"In the U.S., any work created by a federal government employee or officer is in the public domain, provided that the work was created in that person's official capacity."... "Keep in mind that this rule applies only to works created by federal employees and not to works created by state or local government employees. However, state and local laws and court decisions are in the public domain."