The O‘ahu ‘Elepaio (Chasiempis ibidis) is an endangered monarch flycatcher endemic to the Hawaiian Island of O‘ahu. Current information on abundance, distribution, and population trend is needed to help assess the species status and identify areas where conservation efforts can be focused. We used spot-mapping methods with song playbacks to conduct surveys in the Ko‘olau Mountains from 2011–2012, and we used occupancy sampling with repeated visits to estimate detection probability. We detected 545 male and 317 female O‘ahu ‘Elepaio. The detection probability of males was 0.92 ± 0.03 and that of females was 0.86 ± 0.05, yielding corrected estimates of 592 males (95% CI  =  554–630) and 369 females (95% CI  =  327–411). Combined with results of a previous census in the Wai‘anae Mountains that found 192 males and 84 females, the total estimated population of the species is 1,261 birds (95% CI  =  1,205–1,317), consisting of about 477 breeding pairs and 307 single males. The O‘ahu ‘Elepaio has declined in abundance by about 50% since the 1990s, when the population was estimated to be about 1,974 birds. The current geographic range of the O‘ahu ‘Elepaio encompassed about 5,187 ha and has declined by 75% since 1975, becoming fragmented into four larger subpopulations with 100 or more birds each and 12 smaller subpopulations. Rat control to reduce nest predation remains the cornerstone of the conservation strategy for the O‘ahu ‘Elepaio, but variation in forest structure, forest dynamics, and continuing evolution of ‘elepaio nesting behavior are likely to play important roles in determining whether this species can persist.