It is not really fully Open Source. Since they changed the license [1], they adopted a open core model and so most of the new features (UI, reports, etc) are only available as private software.
Also they have a quite strange interpretation of the xGPL license mixed with proprietary license.
Users of their SAAS service should be careful to not install those private software otherwise they will not be able to change of hosting service.
[1] https://www.odoo.com/blog/odoo-news-5/post/adapting-our-open...