U.S. Civil Service Commission investigators yesterday reported that nearly half this city's black police officers saw copies of entrance exams before taking the tests.

The federal investigators, on loan to the city for the duration of the probe, further stated that police Director Grover W. O'Connor knew of cheating, but took no action.

In a 145-page report, the two investigators cited numerous instances of apparent civil service test fraud between June and December 1975, a period in which the city was actively involved in minority recruitment.

They said O'Connor, who was denied any wrongdoing, "could not but help to have [had] knowledge [of the instances]." His officers reported cheating directly to him, they concluded.

The investigation into alleged irregularities in the hiring and promotion of black police officers was ordered by city commissioners in the wake of allegations by former civil service examiner Theodore W. Thompson.

Thompson made allegations about hiring irregularities narcotics transactions in the police department during questioning after he was arrested in connection with a robbery-slaying on May 24.

He was found hanged in his Montgomery County jail cell May 22. The death has been ruled a suicide by the county coroner.

Investigators said polygraph tests show that at least 20 of the city's black police officers lied when asked if they had seen advance copies of the entrance exam.

City Manager James A. Alloway said it was "too premature" to determine what action would be taken in connection with the report's findings. He said a special task force of attorneys would be formed to review the investigators' documentation.

City Commissioner Richard A. Zimmer said some city employes will be disciplined or discharged, based on information from the report.

The investigators also concluded that portions of a sergeant's promotion test had been leaked to certain black officers in digested form.

"The sad irony is that many of those persons availing themselves of such 'help' may well have been qualified without it," they said.

Local black leaders have demanded that the civil service investigation be extended several years before 1975 to see if favors had been granted to any of the department's 403 whites.

The department, meanwhile, has contracted a private firm to investigate Thompson's allegations of drug dealing by police officers.