Is it true that Open Access means an article is not copyrighted?
No. Choosing to publish through an open access channel does not mean the article is not copyrighted.
The same options exist when publishing through an open access channel as when an article is published through a controlled-access (or traditional subscription) model: the author may in some cases be able to retain copyright, or may be required to grant the journal publisher copyright. But in either case, the article is still copyrighted, either by you or the publisher.
There is no direct and clean relationship between open access journals and copyright policy. Many, but not all, open access journals have liberal policies that allow authors to retain copyright. Most traditional subscription-based journals have standard copyright transfer agreements that require authors to turn over copyright upon publication, but some such publishers will agree to negotiate this requirement, or have a standard agreement that is more liberal.
Some publishers of hybrid journals allow authors to retain copyright for articles published under their open access option; others will still ask that you transfer copyright.
Even when self-publishing on the web, the author has copyright to the content. As an author and copyright holder, if you wish to clearly instruct readers about what you authorize them to do with your content, you can attach a creative commons license.