Updated to v3 GPL.

OpenFOAM is licensed under the GNU GPL v3 - consequently work using OF is referred to by their licensing paragraphs on the source code:

OpenFOAM is free software: you can redistribute it and/or modify it
    under the terms of the GNU General Public License as published by
    the Free Software Foundation, either version 3 of the License, or
    (at your option) any later version.