LEK and OC&C Case Interviews: How to Prepare for Number-Heavy Cases

Candidates preparing for LEK and OC&C interviews frequently describe a different texture from MBB-style practice cases: more data on the page, more calculations in a row, and a clock that feels shorter. That description is consistent with the work these firms are commonly associated with, commercial due diligence for private-equity investors and corporate buyers, where the work is often to give an investor an outside-in view of the market, competition and forecast, as one input to whether to bid and at what price.

A statement about what we know and do not know. LEK describes its process only in general terms (rounds that mix case and background interviews, with cases that can be quantitative or strategic), and OC&C's recruiting postings give a short outline that varies by office and year. Neither publishes how cases are run or scored. What follows is built from how candidates commonly describe the process and from the nature of the work, not from official guidance. Where this post says "commonly," it means exactly that. One data point does support the "shorter clock" premise: in recent postings, OC&C has described first-round cases of about 30 minutes. Confirm the length and format for your office and year with your recruiter.

What sits behind the interview

Both firms are commonly associated with work in which a consultancy is hired to test the commercial case for an investment: how large and how fast-growing is the market, how does the target compare with its competitors, how loyal are its customers, and does the management forecast hold up. LEK is also widely associated with healthcare and life sciences, and OC&C with consumer and retail, though sector strengths differ by office.

The relevant point for an interviewee is that this kind of work is quantitative, quick, and conclusion-driven. A diligence team commonly has weeks, not months, to give an investor an outside-in view of the market, competition and forecast, as one input to whether to bid and at what price. It makes sense for interviews to try to show whether you can work that way. Our piece on what private-equity due-diligence cases test covers the structure of that work in more detail.

How number-heavy cases tend to differ

The table below contrasts a typical MBB-style practice case with what candidates commonly report for number-heavy interviews. These are tendencies, not rules; your interviewer may run a case that looks nothing like the right-hand column, and MBB interviewers sometimes grill candidates on math too.

A number-heavy case, at speed

Here is a compressed example of the style: one exhibit, five linked calculations, and a question about what has to be true. It uses the same logic as the Meridian example in our private-equity post, compressed, with a different company and different numbers, and run forward from the market instead of back from a target return.

A fund is looking at a regional commercial-cleaning company. Management says it will grow revenue from $100M to $150M in two years while lifting its EBITDA margin from 12% to 15%. A third-party estimate has the regional market growing 10% a year. Your job is to say what has to be true for the plan to work.

1. Share today. $100M ÷ $2,000M = 5%.
2. Market in year 2. Two years of 10% growth is 1.1 × 1.1 = 1.21, so $2,000M × 1.21 = $2,420M.
3. Revenue if share holds at 5%. 5% × $2,420M = $121M. Cross-check: $100M × 1.21 = $121M, because a company that keeps its share grows with the market.
4. Share the plan needs. $150M ÷ $2,420M ≈ 6.2%. That is a gain of about 1.2 points, or roughly 24% in relative terms (1.2 ÷ 5), in two years. The plan implies revenue growth of about 22% a year (1.5 has a square root of about 1.22) against a market growing 10%.
5. EBITDA gap. Plan EBITDA is $150M × 15% = $22.5M. If share and margin both hold, EBITDA is $121M × 12% ≈ $14.5M. The gap is about $8M, of which about $3.5M comes from extra share (($150M − $121M) × 12%) and $4.5M from the margin gain ($150M × 3 points).

The point is not the answer. A number-heavy interview rewards you for finishing the arithmetic and then using it. The plan needs both a gain of about 1.2 points of share and a 3-point margin expansion, so which is more believable, and what would you ask for? Is the market fragmented enough for a 5% player to take share? Does the margin gain come from scale, price or cost? Practice versions are usually simplified in the same way, with one market growth rate, a constant share assumption and no price changes. Real diligence work is messier, but the "what must be true" logic is the same.

Where chained calculations go wrong

When several calculations feed each other, one early slip contaminates everything after it. The failures are predictable:

• Forgetting to compound. Applying 10% once gives a $2,200M market instead of $2,420M, and the share the plan needs becomes 6.8% ($150M ÷ $2,200M) instead of 6.2%. A gain of 1.2 points turns into one of 1.8 points, and your conclusion changes.
• Mixing points and percent. Share rising from 5% to 6.2% is 1.2 percentage points, or about 24% in relative terms. Say which one you mean, every time.
• Calculating without a plan. State the sequence first: "Market in year two, then revenue at constant share, then the share the plan needs, then EBITDA." It keeps you oriented and gives the interviewer a chance to redirect you before you spend time on the wrong step.
• Never cross-checking. Ten seconds is enough. In the example, revenue at constant share came out at $121M two ways: 5% of the year-2 market, and today's $100M grown by the market's 1.21. If the two routes disagree, you have found an error while there is still time to fix it.

How to prepare differently

1. Build speed without sloppiness

Drill chains of three to five linked calculations on one data set, not isolated problems. Use round-then-correct, convert percentages to fractions, and anchor to percent of revenue; our mental-math shortcuts and worked drills are the toolkit. Then put the speed under a narrator: a fast, silent calculation is hard for an interviewer to follow or to credit, and a wrong one is hard to rescue. The narrating calculations post shows how to say it as you go.

2. Get fluent with tables

Practice reading a table of eight to twelve numbers and stating its point in one sentence before calculating anything. Check units, time periods and whether figures are totals or percentages. Most errors in this style of case come from picking the wrong column, not from bad arithmetic.

3. Link size, growth and profit

Practice sizing a market and then projecting it forward: how big is it, how fast is it growing, what share does the target hold, and what margin does that imply? The market-sizing post covers the sizing half; the example above is the projection half.

4. Learn the deal logic

You do not need finance experience, but you should be comfortable with EBITDA, a valuation multiple, and the idea of a forecast. Learn the levers that drive a forecast (market growth, share, margin and, at the deal level, the multiple paid) and get used to asking which one a given case depends on. The M&A four-lever framework is a related way to organize that thinking.

5. Keep the MBB basics

Number-heavy does not mean communication-light. A clear structure, a stated hypothesis and a firm recommendation still matter, and an interviewer will usually be more forgiving of a slightly slower, well-narrated calculation than of a fast one they cannot follow.

If you are preparing for both worlds

Most of your prep is shared. Structure, synthesis, fit stories and communication transfer directly, so do not split your effort in two. Add a number-heavy block on top: a couple of sessions a week of chained calculations and table reading in the weeks before these interviews, plus a handful of deal-style cases. If you are also comparing these firms with MBB on pay and exit routes, our tier-2 versus MBB comparison lays out the trade-offs; do not let pay assumptions decide where you aim your prep before you know the process at your own school.