Recently, a revolutionary effort to seek fundamental improvement of 802.11, known as IEEE 802.11ax, has been approved to deliver high efficiency wireless local area network (HEW) technologies for dense scenario. The de-facto random access mechanism of IEEE 802.11ax is uplink OFDMA-based random access (UL OFDMA RA) mechanism. In this paper, we provide a simple but accurate analytical model to study the system efficiency and delay performance of UL OFDMA RA under saturated condition, in the assumption of a finite number of stations and ideal channel conditions in a single-hop WLAN. Simulations are conducted to validate analytical results. In addition, we use this analytical model to identify the appropriate OFDMA-based random access parameter set (RAPS) that can maximize the system performance, a feature which is essential for system design and configuration.