This paper investigates the mobile user association problem for heterogeneous networks. More specifically, it applies the optimal transport (OT) concept to determine the cells corresponding to each base station (BS) that minimize the total transmit power consumption of orthogonal frequency-division multiple access (OFDMA) cellular networks. This is performed by respecting the network quality of service and by taking into account the resource limitation per BS (e.g., power budget and number of resource blocks). Starting from given BS locations and user distributions, a fixed point algorithm is employed to find the optimal solution for the formulated problem. Selected numerical results investigate various practical scenarios with different user distributions and show that the cell boundaries obtained using OT solution provide a significant energy saving compared to the classical Voronoi cell boundaries.