Providing on-demand video streaming services to large numbers of nodes in mobile wireless networks is a challenging task. We propose MOVi Mobile Opportunistic Video-on-demand, a mobile peer-to-peer video-on-demand application based on ubiquitous WiFi enabled devices such as smartphones and Ultra Mobile PCs. MOVi poses new challenges in on-demand video streaming caused by limited wireless communication range, user mobility and variable user population density. MOVi addresses these challenges by exploiting the opportunistic mix use of downlink and direct peer-to-peer communication to significantly improve achievable overall system throughput. Through simulation and actual implementation, we observed that MOVi is able to increase the number of supported concurrent users two fold compared with unicast based on-demand video streaming, as well as reducing video start-up delay by half. Significantly, these performance improvements can be achieved without increasing power consumption at the end device. Using MOVi, users not only share and distribute on-demand video such as YouTube clips for their own performance gain but also for the benefit of the collaborative group.