In this research, we have shown that the medium access control protocol overhead and collisions in contention control messages play a significant role in the performance issues of IEEE 802.11, especially when the number of contending devices is fairly high. To improve its performance, multiuser dynamic OFDMA based IEEE 802.11 DCF has received significant interest from the researchers in recent time. After carefully considering the major drawbacks of the existing research works that have incorporated OFDMA at the physical layer, we have proposed a novel OFDMA based IEEE 802.11 enhancement with a complete DCF. By dividing the channel into the optimum number of sub-channels to allow disjoint groups of devices to simultaneously contend for channel access, our DCF can allow multiple concurrent transmissions or receptions to reduce collisions and consequently improve the throughput and delay significantly. We have developed a complete analytical model of our proposed DCF and demonstrated the effectiveness and superiority of the proposal for access point based operations of IEEE 802.11 in a single cell saturated network. Finally, we have extended our work to support ad hoc operations.