Acoustic sampling for occurrence of the endangered Hawaiian hoary bat (Lasiurus cinereus semotus) was conducted at 12 locations on U. S. Army installations on O‘ahu Island, Hawai‘i. Bats were confirmed as present at 10 of these locations: Dillingham Military Reservation, Helemano Military Reservation, Kahuku Training Area, Kawailoa Training Area, Mākua Military Reservation, Schofield Barracks East Range, Schofield Barracks West Range, Schofield Barracks (Mendonca Park Housing), Tripler Army Medical Center, and Wheeler Army Airfield. Our acoustic sampling did not record bat vocalizations at Fort DeRussy or Fort Shafter. Despite the presence of bats at the above 10 locations, foraging activity as identified from characteristic feeding buzzes was observed only at East Range and West Range of Schofield Barracks. Nevertheless, Hawaiian hoary bats were recorded actively searching for prey in airspace at 10 of the 12 areas during important periods of Hawaiian hoary bat life history, including periods of pregnancy, lactation, and pup fledging. Within-night bat activity pooled for all nights and detectors at each location showed bat activity was mostly confined to the first several hours of the night. This acoustic study detected bats at lower rates of occurrence (frequency of detection [“f”] = 0.07) compared to detection probabilities (“dp”) observed on the islands of Hawai‘i (dp = 0.56) and Maui (dp = 0.27), implying either behavioral differences or that they occur at lower densities on O‘ahu. The rate is also consistent with results from two previous acoustic studies conducted on O‘ahu; a year long monitoring study in the northern Ko‘olau Mountains in 2014 (dp = 0.08), and short-term seasonal Army monitoring efforts in 2012 (dp = 0.05 to 0.06).