The effect of oxygen content and Ti addition on the glass forming ability (GFA) and crystallization kinetic of Zr-based bulk glass forming alloys have been studied by means of thermal analysis and X-ray diffraction techniques. Presence of oxygen triggers the formation of a nanocrystalline metastable f.c.c. Zr-2 Ni-type phase which can act as heterogenous nucleation sites for the formation of dendrites during solidification. An increase in oxygen content changes the crystallization behaviour from a single- to a double-step process and triggers the crystallization of stable Zr-2(Cu,Al) besides metastable Zr-2 Ni-type phase. Oxygen-triggered nucleation of a nanocrystalline metastable Zr-2 Ni-type phase is found to be the initial step of crystallization. The important parameters of GFA such as glass transition temperatures, T-g, the crystallization temperatures, T-x, and crystallization enthalpies, Delta H, were determined by using DSC. It was observed that the presence of oxygen, even in a very small amount, and Ti addition cause a drastic reduction of the supercooled liquid region, Delta T-x, accompanied by a change of the crystallization kinetic. This leads to the decrease in stability of the supercooled liquid, consequently results in a deterioration of the glass forming ability of the alloy.