Happy Birthday - S5.E26

It's Oliver's birthday, and everyone but him wants to celebrate. It also (coincidentally) happens to be Arnold's birthday too. Everyone tries to persuade Oliver to get Arnold a birthday present, while he thinks he deserves one because he's human, while Arnold isn't. It turns out Arnold, and Lisa are the only one's who get Oliver a gift, and when he and Lisa go over to the Ziffel's house for his party, only to find that there wasn't one, they return to find all of their furniture stolen.