The Wharton MBA/ Lauder MA program is my dream program and I understand where you are coming from on wanting to know the format of the OPI.
I have not taken the Lauder OPI, but I can tell you a logical format from my experiences being trained in Russian at Defense Language Institute. The OPI will begin with some short banter or conversational greetings. The examiner will get a topic from this conversation and ask you about it. If you haven't performed well, the teacher will keep the conversation light, and give you a basic or intermediate level. If you haven't made and significant grammatical errors and you sound like you know what you are talking about, the examiner will probe you with more difficult questions.
Think of it as similar to the CAT test GMAT. The OPI adjusts with your abilities. In the military version, if you messed up at the beginning, you couldn't get past a certain score. You can't prepare for the specific questions because the type will change, but the relative difficulty will not.
I am not sure that the Lauder OPI is the exact same format, but I listened to the Russian Advanced recording and it seemed to me to be very similar to what we did.
Good Luck.