The O'ahu 'Elepaio (Chasiempis ibidis) is an endangered forest bird endemic to O'ahu and has declined steadily during the past century. Current information on distribution and abundance is needed to help assess the species status and identify areas where recovery efforts can be focused. We used spot-mapping methods to census O'ahu 'Elepaio in all suitable forest habitat in the Wai'anae Mountains from 2006 to 2010 and compared results with previous surveys from the 1990s. We detected a total of 300 O'ahu 'Elepaio, including 108 breeding pairs and 84 single males. The sex ratio was strongly male biased due to nest predation on females. Their distribution was extremely fragmented, and the only concentrations were in 'Ekahanui (38 pairs), Schofield Barracks West Range (40 pairs), and Pālehua (15 pairs). We failed to detect 'Elepaio in many areas where they were observed in the 1990s. 'Elepaio have become more sparse in other areas, indicating that they are continuing to decline. Nest predation by alien black rats (Rattus rattus) and mosquito-borne diseases are the greatest threats. Rat control programs have helped reduce nest predation and stop declines in several areas, but only a fraction of remaining 'Elepaio benefit from active management and further declines can be expected unless rats are controlled on a larger scale. Alternative methods of rat control should be explored, and restoration of native trees that are less attractive to rats might provide safer nest sites and reduce the need for rat control.