Apatite-type phases have been attracting interest as a new class of oxide ion conductors, and in these systems, the key defects are oxide ion interstitials. We have recently been investigating Y doping in La9.33+xGe6O26+3x/2 and have shown that up to 30% Y incorporation helps stabilise the hexagonal lattice. However, for higher Y levels, i.e. La6Y4Ge6O27, a triclinic cell is obtained. Such samples also show a significantly lower conductivity at low temperatures. However, at temperatures >750C, a significant enhancement in conductivity is seen typical of an oxygen order-disorder transition. To our knowledge this is the first example of such a transition in an interstitial oxide ion conductor, and the aim of this experiment is to investigate its' nature.