congrats to making it to interview stage. Here a few thoughts..
1. Case interview format: Typical at OW, you'll mostly find 45min case interviews with quite standard agenda - 5-10min short intro / fit questions → 25-30min case → 5min your questions.
2. Style of cases: tends to be more candidate-led but not exclusively (really depends on the specific interviewer). Cases are fairly heavy on sizing + quant, especially in R1. Occasionally, some of my OW coachees got brainteasers in the past (one of them at Milan office).
3. Team Assessment: yes, from what I've seen with local office, it's typically a case to solve. Ask to evelop a market entry strategy, solve profitability problem etc are examples I've seen - for candidates well-prepped in casing, nothing to worry about, but the teaming aspect & dynamics become important here.
Hope this helps - feel free to reach out via PM, if you'd like to learn more.