The Khmer Rouge tribunal sentenced Duch to a commuted 19 years in prison, earlier this year.
More than 8,000 victims have filed with the unit and are awaiting the trial of four jailed senior Khmer Rouge leaders.
In a December report, OSJI says donors can push harder for wider indictments and great participation by leaders.
Siegried Blunk, of Germany, will replace Marcel Lemond, who resigned in September.
It has been screened for professors of political science, human rights, law, genocide and Southeast Asian studies over the past weeks.
Chams suffered greatly under the Khmer Rouge, and their plight in part makes up the charges of genocide against the four leaders.
The observer dismisses the transferring of cases to local courts as a plausible solution to the court's financial problems.
As many as 200,000 people were forced into marriage under the Khmer Rouge, which sought a communist agrarian utopia.
Duch was arrested in May 1999 and was later charged by the tribunal for war crimes, crimes against humanity, torture and homicide.
A leading Khmer Rouge tribunal monitor said Monday he was optimistic about the UN-backed court.