In this paper, we propose an energy and cost efficient WDM/OFDMA PON system architecture, referred to as NEWOPS. Based on a two-tiered semi-tree topology, NEWOPS allows a large number of ONUs to be connected to OLT using only a handful of colorless transceivers. NEWOPS' unique architecture not only facilitates high spectral efficiency and flexible sub-channel bandwidth allocation, but also enables the system to achieve high energy and cost efficiency. We conduct a theoretical study to identify and resolve a new data-rate performance issue due to the colorless and wavelength re-use design of the system. Experimental results show that when only 5-GHz bandwidth is allotted for each wavelength, NEWOPS successfully achieves symmetric 20-Gb/s downstream and upstream OFDM-16QAM transmissions via the same fiber trunk. As a demonstration of our system, we show how NEWOPS achieves energy efficiency that is two to twelve times higher than that of a typical tree-based WDM/OFDM PON when supporting 40 ONUs.