In a decision that could have widespread implications for other federal agencies, the Army yesterday dropped its opposition to federal occupational safety inspectors investigating complaints at many of its installations.

Army Secretary Clifford L. Alexander Jr. issued instructions to all Army facilities yesterday that any use of civilian contractor employes would be open to inspectors from the Labor Department's Occupational Safety and Health Administration, the agency charged with inspecting most private workplaces.

OSHA officials said they were still studying the instructions. But the Army's decision is likely to weigh heavily on other federal agencies such as the Navy and the Department of Transportation which have resisted OSHA inspections.

Technically OSHA has been upheld in the courts in its assertion that it has the right to inspect federal installations where private contractors are used. But federal agencies such as the Navy, which is under fire for allowing exposure of workers at its shipyards to the carcinogen asbestos, have sharply resisted efforts by OSHA inspectors to look into worker complaints.

OSHA officials have estimated that the lack of outside inspection by federal occuptional safety and health investigators could be costing the government as much as $5 billion in accident and property damage costs annually.

The Army's decision came after mounting pressure on its Material Development and Readiness Command (DARCOM) to allow OSHA inspectors to investigate complaints of lax safety conditions in ammunition plants owned by the Army and contracted out for operation to civilian firms.

At the Radford, Va., Ammunition Depot, DARCOM officials and the Hercules Corp., which operates the plant for the Army yesterday allowed two OSHA investigators onto the sprawling plant grounds after having turned them away three times earlier this year.

The plant has been the site of three major explosions and a serious fire since 1971. Workers there complained recently to OSHA that high levels of benzene, a cancer-causing chemical, had been identified in the air of the plant in the past and that benzene was being used there again this week.

A spokesman for the Radford facility said yesterday that benzene had been used last year in manufacturing propellant for the Sidewinder missle but said that use of the chemical had been discontinued after May 1977. OSHA had declared then that the chemical was a carcinogen and set worker exposure limits of one part per million in the plant's air.

The Army spokesman denied allegations by the Oil, Chemical and Atomic Workers Union yesterday that the Army and Hercules had not told workers of findings of up to 700 parts per million of benzene in the air during manufacturing operations last year.

But he said that the Army ammunition facility will not use benzene in future manufacturing operation because it is considered too dangerous. The clear, liquid chemical has been linked by federal scientists with the development of leukemia in some persons exposed to it.

Until Alexander's decision yesterday, the Army had resisted efforts to get OSHA inspectors into the Radford plant and the 13 other ammunition facilities the Army owns. The Army's reluctance to admit OSHA to the plants contradicted a written instruction earlier this year from the office of the Secretary of Defense granting OSHA access to the facilities.

The order by the Army secretary does not extend permission to OSHA inspectors to investigate complaints directly involved with explosive manufacturing. However, Basil Whiting, deputy assistant secretary of labor for OSHA, said yesterday he would seek authority to get OSHA into all aspects of the ammunition manufacturing process and to defer certain enforcement areas to the Army.

"We want to examine all potential hazards and then we will defer to the Army's statutory authority to make sure there are no gaps in the coverage protecting workers," Whiting said.

He said Army facilities using civilian contract employes would be immediately placed on OSHA regular inspection schedules. "We will also respond to any complaints we get from workers in these plants and facilities," he said.