The OpenSceneGraph is protected by Copyright, and is licensed as OpenSource, so the source code is freely available, modifiable and redistributed under the terms of the OpenSceneGraph Public License (OSGPL). The OpenSceneGraph Public License is based on the Lesser GNU Public License (LGPL) and includes the !wxWidgets additions to LGPL which provide a clear statement that distribution with proprietary applications is accepted.
The principles behind the OpenSceneGraph license entitles you to freely modify and copy the source code and libraries built from the source code under the terms of the license. You may not distribute the source code or libraries under any other license, unless authorised to do so by the Copyright holders. If you do not accept all the terms of the license then you do not have the right to copy the source or libraries, such copying is illegal under the terms of international Copyright law.
Like most software, be it closed or open source, the OpenSceneGraph is provided as is, without any warranty of any kind, or fitness for use, no indemnification is provided. By accepting the license, you accept these disclaimers.
The OpenSceneGraph Public License is designed to be compatible with use of proprietary applications, such applications can statically and dynamically link to the OpenSceneGraph and can be redistributed with the libraries free of charge. Your applications can be distributed under any license, be it open source or proprietary.
Modifications to the source code of the OpenSceneGraph must remain under the OSGPL and under the Copyright of the original code, this covers bug fixes or feature enhancements to the source code.
User written extensions to the OpenSceneGraph, such as subclassing from OpenSceneGraph classes, are not covered by the OSGPL, and you are free to license these extensions as you wish, under a proprietary or open source license.