Our software is released under the GNU GPL
(specifically version 2). The GNU website has plenty of information what you can and can't do with software using its license, as does Wikipedia etc.
OJS also includes optional
subscription/content access controls, which have absolutely nothing to do with the software license itself -- the software license only applies to the software code, not the material published using the software. (I hope that's clear as mud.
In a nutshell, you (or any publisher) are absolutely free to use our software to publish open access or subscription content; you're free to modify the software, retheme it, extend it, etc. and even free to redistribute it provided you keep the original attribution and license in place. You're also more than welcome to install the software yourself and charge a hosting service for others; or offer OJS development services. The major problems we've encountered have almost all involved folks rebranding/reselling OJS as a software product of their own, without proper attribution to PKP, and while also removing or rephrasing the software license. So basically, don't do that and you'll be all set.
If you have any other questions, let us know!