All of the art hosted on OGA is free to use, including in commercial projects. However, each piece of art is released under a particular (or perhaps many) license. Different licenses have different conditions, and if you use art from OGA, you must be careful to follow the terms of one of its licenses. None of the licenses that OGA accepts prevent you from modifying and using art, including in commercial projects, however some have conditions which you may want to avoid as a commercial developer.

For instance a recent submission http://opengameart.org/content/game-backgrounds-0.

On the left panel, you can see that it is released under the following licenses:

CC-BY 3.0

CC-BY-SA 3.0

GPL 2.0

GPL 3.0

OGA-BY 3.0

If you use that art, you may select ANY ONE of those licenses, and safely ignore the others. Each license has its own conditions, which are described briefly in the FAQ, and in more detail by Capbros here. To my knowledge, nobody affiliated with OGA is a lawyer, and our understanding of the license requirements is often not clear, and may be innacurate. It is your responsability to ensure that you meet the license requirements.

@Ramert1404:  If you have the time, it would be great to hear if you found anything in the FAQ or my suggested changes useful or informative, or if you had any further questions after reading them.