Recently, IEEE 802.11 started a task group, 802.11ax, to investigate and deliver next generation wireless local area network (WLAN) technologies for dense environments. In 802.11ax, orthogonal frequency-division multiple access (OFDMA) has been adopted as PHY layer protocol to support up-link multiple-users (MU) transmission. However, choosing multiuser PHY in WLAN creates a new challenge as the legacy random access protocol, carrier sense multiple access/collision avoidance (CSMA/CA), was designed for single-user PHY. Hence, designing a new random access protocol for 802.11ax medium access control (MAC) is desperately needed. In this paper, we firstly introduce 802.11ax MAC and its random access protocol. Theoretical limitation of system efficiency of 802.11ax random access protocol is also provided. Next, we discuss the most popular contention resolution algorithm proposed in the 802.11ax task group at present. Finally, we propose a pragmatic and efficient OFDMA-based hybrid channel access (OHCA) method for IEEE 802.11ax. To the best of our knowledge, this is the first paper that study the 802.11ax random access protocol and its system efficiency analysis. Simulations are conducted to evaluate the performance of the proposed scheme. The results show that the proposed scheme provide much better performance than the existing solution, offering low collision rate, short access delay, better fairness, and yet more efficient bandwidth usage.