Efficient support of multimedia services in next generation wireless networks requires advanced scheduling schemes that achieve both system capacity maximization and full QoS differentiation. This paper proposes a new MAC scheduling scheme for multiuser OFDM wireless networks. Designed in an opportunistic cross-layer approach, this scheme uses a system of weights that dynamically adjusts the priority of the flows considering the transmission conditions and the QoS that they currently experience. This results in a very efficient management of the bandwidth. Performance evaluation shows that the proposed scheduling outperforms existing wireless OFDM based scheduling schemes providing full QoS differentiation, high fairness and system throughput maximization.