Ox and all its components are copyright of Jurgen A. Doornik. You have permission to pass on to others those components which also can be publicly downloaded via my work page (e.g. oxbas110.zip, oxsun110.zip, oxlnx110.zip, arfima.zip etc.). This should be done in its original form. These may be used freely for research and teaching purposes; the Windows version is not free. Commercial users are expected to purchase the Windows version of Ox and GiveWin with documentation, regardless of which version they use.
If you redistribute modified versions of code written by me, you must clearly indicate the original source, and take over the responsibility of supporting that code. Remember that often new functionality can be implemented by deriving from an existing class (instead of adjusting the base class).
The Ox syntax is public, and, of course, you may do with your own Ox code whatever you wish. If you make your code available to others, I would appreciate it if you could provide proper documentation (such as, for example, in the Arfima package). Also, try to avoid global variables: either wrap the code in a class, or hide the globals with the static keyword, providing functions to set the values.
You may not charge for any of the components which are made available by me. Contact me if you wish to distribute Ox with other products, or, for example, include it on a CD. Several Ox versions on Unix platforms are compiled with GCC. I have problems fully understanding the copyleft agreement, but I think I comply: you may pass on oxlnx110.zip (say), and you can relink oxlinux with other versions of the libraries.