Medical experts said her current mental state would prevent her from fully participating in a trial, even by video link.
A UN spokesman said last week O’Brien plans to meet political and court officials, including Council Minister Sok An.
Peou Nam was taken twice for execution in a pit by the Khmer Rouge, and survived both times.
Chhang Youk, added in a later interview that the UN should not allow the resignation to go uninvestigated.
The hybrid court, the first of its kind, has spent $150 million and so far completed only one trial since 2006.
Government spokesman Khieu Kanharith dismissed allegations the government was interfering with the work of the court.
His resignation comes a week after Human Rights Watch said he and his Cambodian counterpart should quit.
Meas Muth, the former commander of the Khmer Rouge navy, is among five suspects in two cases before the UN-backed tribunal.
The group said judges Siegfried Blunk and You Bunleng had failed to conduct genuine, impartial and effective investigations.
Van Schaack says the lack of post-Khmer Rouge transitional justice efforts explains current persistence of high social trauma.
Im Chaem spoke to VOA Khmer in an interview at her home in Anlong Veng district, Oddar Meanchey province, in July.
The judges recently rejected the applications of at least three people seeking to participate as victims.