I've gathered some info on OW and what I can say is they are much less structured when compared to McK and others.
1) Sometimes they don't even ask for structure;
2) I've heard from friends that went to final rounds who got a lot of "strange" estimation cases (PM me if you want to learn a few of them)
3) The style of case depends a lot more on the interviewer rather than the firm (in other words, depending on the interviewer you will get a more interviewer-led or candidate-led)
4) The cases tend to be shorter (this could depend from country to country) - the entire case is more 40-45 minute, with a good 7-10min FIT and at least 5-10min Q&A so the case usually is from 25-35 minutes (again, this can change from office to office)
Hope this helps