Ajou, a 7-foot-2 center who is the tallest player in the program's history, was declared by the NCAA to be eligible for his first season.
During Northwestern's limited summer practice time, Ajou was not able to join his new teammates on the court, with his eligibility pending.
A native of Sudan, Ajou played two years with Culver (Ind.) Academy after attending prep school at St. Thomas More School in Connecticut.
Northwestern's 2012 recruiting class also includes Olah, three-star forward Kale Abrahamson, and three-star guard Sanjay Lumpkin. Louisville transfer Jared Swopshire also joined the group, and is eligible to play this season.
Ajou was added to Northwestern's class in June, the weekend in which the freshman class moved onto campus. He averaged 10.1 points and 6.3 rebounds per game during his senior season.