Simple math: government has three branches (executive, legislative and judiciary) and roughly two levels (national and sub-national). Hence, at least in theory, there is a minimum of six institutional settings that are the object of “open government” (see table below).
As far as I know, the Open Government Partnership (OGP) has privileged its activities in only one of these spheres, that of the national executives (area 1). In this respect, it appears that one of the next challenges for the OGP is to find ways to involve the remaining areas (2, 3, 4, 5 and 6).
From an international standpoint, some might even – and reasonably so – claim that it is precisely areas 2, 4 and 5 where the strongest potential for open government actually lies, and where most innovations in the field actually happen.
As to the judiciary (3 and 6), one could easily assert that it is the branch of government in most dire need of openness, as in most countries these are literally opaque institutions.