Chris Klein stated in an interview with Huffpost that Oz, Vicky and Heather were not considered for the movie and that he didn't refuse to reprise his role as many believe.
On the first American Pie: Wie ein heißer Apfelkuchen (1999) film, Eugene Levy was only required for 3 days of filming. For this sequel, he was on set for 35 days.
This is the only film in the series where the guys aren't seen eating at Dog Years. The closest reference to the eatery is when Stifler is seen drinking from a Dog Years cup at the sporting goods store.
In a deleted scene Jim explains Oz's absence from the wedding saying he was still in Spain with Heather, another absent character.
Out of all the characters who were missing from this film, Nadia is the only one who received an honorable mention.