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In the wireless LANs or mobile ad hoc networks, a node with multi-packets in its queue waiting for delivery to several neighboring nodes may choose to schedule a candidate receiver with good channel condition for transmission. By choosing a receiver with good channel condition, the Head-of-Line (HOL) blocking problem can be alleviated and the overall system throughput can be increased. Motivated by this observation, we introduce the Opportunistic packet Scheduling and Media Access control (OSMA) protocol to exploit high quality channel condition under certain fairness constraints. We base our design on CSMA/CA so that it can be simply incorporated into the 802.11 standard. The key mechanisms of OSMA protocol are multicast RTS and priority-based CTS. In the OSMA protocol, RTS includes a list of candidate receivers. Among those who are qualified to receive data, the one with the highest order would be granted to catch the channel by replying CTS in the first place. The ordering list is updated dynamically according to certain scheduling policy such as Round Robin (RR) and Earlier timestamp First (ETF), and also other performance metrics, ex., fairness and timeliness, can be enhanced. To the best of our knowledge, this is the first paper to exploit the multiuser diversity in the CSMA/CA based wireless networks. We evaluate the OSMA using ns-2 and our simulation results show that this protocol can improve the network throughput significantly.