OJS is licensed under the GNU General Public License v2
, which is also included with the application (see docs/COPYING). Basically (please take the license's terms over what I state here) it states that you are free to use the application as you like, but if you re-distribute it, you must also redistribute the source code you're using under the same terms. It doesn't prohibit commercial use. It's not strictly required, but we would appreciate it if you gave credit of some sort to PKP for providing and maintaining the software. Simply hosting and selling services using OJS, as far as I'm aware, won't trigger the redistribution terms in the GPL. There are several providers doing this.
Public Knowledge Project Team