The effect of large concentration of oxygen on the microstructural development of Zr-Cu-Al-Ni bulk metallic glass (BMG) alloys, prepared from commercially available Zr sponge, has been studied. Apart from promoting crystallization, increased concentration of oxygen (similar to 8000 ppm) spawns additional phases. In particular, we report the appearance of alpha-Zr, dendritic Zr2Cu phase and the Zr2Ni type cubic phases. Addition of oxygen scavenger like Yttrium only partially solves the problem. Phase evolution was also found to be sensitive to the cooling rate and hence to the thickness of the cast sample. Thus it is possible to produce a gradient microstructure with predominantly amorphous phase at the outer layer.