We fabricated TiO2-ZnO core-shell nanofibers via a novel two-step process. In the first step, the TiO2 core nanofibers were synthesized by electrospinning. Subsequently, the ZnO shell layers were grown in a controlled manner using atomic layer deposition. The methodology proposed in this work is expected to be one of most suitable methods for preparing various kinds of oxide core-shell nanofibers or nanowires. We investigated the O-2 sensing properties of the synthesized core-shell nanofibers. Good sensitivity and dynamic repeatability were observed for the sensor, demonstrating that the core-shell nanofibers hold promise for the realization of sensitive and reliable chemical sensors.