A journal or article labeled "open access" at an open access site is intended to be freely accessible on the web for viewing and sharing, with no cost incurred by the viewer. However, that does not mean the item is necessarily copyright free.
Authors and publishers can retain some rights to their open access works. For instance, authors will often retain control over the integrity of their completed work and the right to be properly acknowledged and cited.
Like with other open access items or content objects, authors can choose to indicate more specifically what rights they retain or choose to give up. Sometimes they do this by using a creative commons license when publishing an open access item. However, if there is no such indication of a creative commons license, or other such indicator, the assumption should be that the author or publisher still retains copyright, even if the item is allowed to be accessed for free.
Some open access sites make it mandatory for all contributors to adopt certain open access usage policies, for consistency. For example, the DOAJ requires all contributors to allow viewers to "read, download, copy, distribute, print, search, or link to the full texts of these articles," making clear the usage permissions.