I'm not sure. I'm certain you'd be able to find a definition of what constitutes the Prime Minister's Office somewhere on the PCO website--the PMO website, for instance. But I don't think that office is created by statute, as are government departments and the commissioner's office, for instance, which is created by statute in the Lobbying Act. So that's a difference.
On October 27th, 2009. See this statement in context.