We report an unusual "symbiotic" behavior of oxygen interstitials acting in concert with Ge and Si interstitials at high temperature inducing morphology changes and autonomous migration of Ge nanocrystallites within SiO2/Si3N4 layers. The Ge nanocrystallites were originally generated by the selective oxidation of SiGe nano-pillars grown and lithographically patterned over buffer Si3N4 layers on Si substrates. The coalescence and movement of these Ge nanocrystallites appear to be very sensitive to the presence and flux of oxygen interstitials especially at the Ge nanocrystallite/buffer Si3N4 interface. A range of different morphologies are observed for Ge nanocrystallites that are directly attributable to the influence of oxygen interstitial concentration and consequently the interstitial Si and Ge concentrations. In combination with Si and Ge interstitials, oxygen interstitials activate the coalescence of sparsely-distributed Ge nanocrystallites and concurrently their migration towards the source of Si interstitials, i.e. the buffer Si3N4 layers, through catalytically-enhanced local decomposition and subsequent oxidation of both the SiO2 and Si3N4 buffer layers. We also show that these symbiotic effects are "tunable" by increasing the Ge content of the SiGe nano-pillars. Dense distributions of Ge nanocrystallites generated from high Ge content SiGe nano-pillars remain static and they show no changes in their morphology possibly because oxygen interstitials are simply unable to penetrate these clusters and consequently incapable of inducing symbiotic Si and Ge interstitial generation.