To improve the spectral efficiency and extend the coverage of next-generation cellular networks, several technologies are proposed, and one of such technologies is mobile relay stations (MRSs). In the system with MRSs, some of mobile stations (MSs) can act as relay stations (RSs) that forward other MSs data from/to the base station (BS). Compared with the fixed RSs, MRSs have several advantages such as low deployment costs and high flexibility. In this paper, we study an opportunistic subchannel scheduling in orthogonal frequency division multiple access (OFDMA) cellular networks with MRSs. We formulate a stochastic optimization problem to maximize the sum-rate of the system with MRSs while satisfying the minimum average data rate requirement for each MS, and then develop an optimal opportunistic subchannel scheduling algorithm by solving it. Through simulation results, we show the performances of our algorithm.