Constructed during the mid to late 1930s on the site of a C.A.S.L. Emplacement. OSO was to have a Twin 6 Pounder Gun, three Dispersed Beam Searchlights and a Director Tower to house fire control and the S.D.L.S. for the lights. There would be a Magazine close by shielded from the sea by a blast wall. Access to OSO was by means of two staircases, one from the Siloso Point sub-surface complex and the other from a path on the surface.
When the above photo was taken, an 8 Inch BL Gun was mounted in the gun emplacement. The camouflage scheme is not the original one, and perhaps more suited to the desert. The scheme in the photo would highlight OSO for any attacker.
OSO stood complete, but unarmed in 1941. A shortage of Twin 6 Pounders led to the gun emplacement being modified for an old 12 Pounder QF Gun. This gun was brought across Blakang Mati from Berhala Reping, which had been rearmed with two Twin 6 Pounders.