Delta-5-9 was a holosuite program that was the stress reaction test part of the Starfleet Academy Preparatory Program. Miles O'Brien initiated the program for Nog in 2371, as the latter attempted to gain entrance into Starfleet Academy.
He thought this would be a runabout simulation; however, he soon learned that in the program, he would be an operations officer on duty in Deep Space 9's Ops. Part of the stress involved in the program was about being tested on what he had not practiced, as explained to him by Miles O'Brien, who was supervising the testing. In this scenario, the trainee had to determine and resolve what caused an apparent explosion on or in the station. (DS9: "Facets")