USS O'BANNON and the initial early FLETCHER class units had the original planned two sonar systems installed side-by-side in sea-chests on either side of the keel. They were near the bow at Frame 25. The original installation was like the pre-war destroyers where the transducers were lowered into the water for operations. However, use of the sonar was limited to lower speeds. To allow operation at desired operational speeds, a dome was installed on ONE of the sonars, the one on the portside. A study was done to see if a larger dome could be installed over both sonars, but was found to be too large and with too much drag penalty. Because of the huge demand for sonar equipment on ASW vessels of all types and the difficulty of producing the unique crystals used in sonar systems built at the time, in August 1942, the USN ordered that only one sonar system would be installed in Fleet Destroyers, but the other sea-chest would be retained and a plate installed over the opening. Destroyers that had the both sonars installed and were still being fitted out or would be going through post-shakedown yard period, had one sonar system removed (the starboard side) and plated over and a dome installed. Even with two sonar systems, the portside sonar had a dome installed over it.
USS O'BANNON would have been one of the few destroyers that became operational and actually were sent to the Pacific, before the one sonar only directive went out, had two sonar systems with a dome over the portside unit. There were several different types of sonar domes installed during the war, but in scale models, they all would look very similar. The images below show the sea-chest, its location, and several images of typical sonar dome installations. The bottom most image, was a mid-to-late war dome type being longer. The object sticking out in front of the dome in the first installed dome image, was a retractable calibration device. During operational use it would have been retracted.