Effects of ozone doping on the Z-shaped explosion boundary of stoichiometric hydrogen-oxygen mixtures are computationally studied. Results show that with increasing ozone doping and within a small range of increment, the explosion limit transitions from the Z-shaped response with two turning points to responses exhibiting four, six, four, two and none turning points. By modifying the rate coefficients of the sensitive reactions within the ozone sub-mechanism, four reactions are identified to control the above highly non-monotonic behavior, and that the system transitions to the H-2-O-3 chemistry as the limit becomes monotonic and overall more explosive. The practical implication is that there exists a critical range of ozone doping such that the mixture is more sensitive to unstable burning for lower levels of doping, with the associated multiple transition states, while the facilitating role of ozone catalyticity is fully realized with just slightly higher levels of doping, resulting in overall stronger explosive burning. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.