Not familiar w/ OC&C; if they are like most consultancies however, they will understand you do not have a business background and won't assume your knowledge in that area is extensive. Still, you are right to want to become familiar with a few concepts. Here are some that should be easy to grasp:
1. Difference between assets & revenue; cash & cash flow; balance sheet & income statement
2. HIgh level understanding of profitability driver. Think here of the structure Revenue = (Price x Quantity) - (Fixed Cost + Variable Cost)
3. Know how to calculate the break-even
4. What type of divisions are in a typical company: Finance, Marketing, Operations / Back Office, Custoemr Service, Legal, Communication. What does each do?
5. General business environment knowledge, read the Times & whatever other economics paper you get your hands on
Last but not least - have you prepared case interviewing? If not, I strongly suggest you kick it up into gear and get busy on this. This will be the most important skill you can display, by far. Victor Cheng's 6 hours of YouTube video is a good place to start, PrepLounge also has some very solid material. Given how little time you have left, I'd also strongly suggest you work with a coach: doing cases with fellow candidates will not be enough without pointed and actionable feedback adapted to your specific needs.
Congrats on scoring the interview, good luck now in the prep.