I had the same experience when I interviewed with OW – I interviewed with the country MD during the 1st round.
You should prepare in the standard way: fit, case and prepare your questions at the end. In my specific case, the interview was purely based on fit questions.
With partners, you may expect that they:
- Spend more time on fit questions and your alignment with the company
- Check more closely your communication (eg how you react to challenging questions)
- May not have a “proper” structured case to present.
So in order to prepare, I would concentrate on the following:
- Review in detail your fit stories. As mentioned, in my MD interview I had exclusively behavioral questions
- Work on your communication (reaction under pressure, how to gain time when you do not have a structure ready, connect with the interviewer, etc). This is something you can do almost exclusively in interviews with peers.
- Prepare cases as usual. OW specifically tends to have more market sizing than other companies