How to Get a Job at Old Mission Capital

Old Mission Capital is a proprietary trading firm specializing in market making across equities, ETFs, and options. Headquartered in Chicago, the firm is relatively small compared to giants like Citadel Securities or Jane Street — typically employing around 100-200 people — but it punches well above its weight in terms of trading volume and profitability. Old Mission is one of the most significant ETF market makers in the United States, providing liquidity for hundreds of exchange-traded products and helping ensure that these instruments trade at prices close to their fair value.

The firm's business model centers on quantitative pricing and efficient execution. Old Mission builds sophisticated models to determine the fair value of securities in real time, then quotes bid and ask prices around that fair value to earn the spread. The firm manages risk dynamically, hedging its positions continuously to maintain market-neutral exposure. This requires advanced pricing models, high-speed technology infrastructure, and traders who can make rapid decisions under uncertainty. Old Mission's systems are built to process market data and execute trades at extremely low latencies, competing directly with the fastest firms in the industry.

What distinguishes Old Mission from larger competitors is its focus and selectivity. Rather than spreading across every product and market, the firm concentrates its resources on areas where it has the deepest expertise and strongest competitive advantages. This focused approach — combined with a small, highly talented team — allows Old Mission to achieve exceptional per-capita profitability. The firm's traders are deeply involved in both strategy development and risk management, creating an environment where everyone understands the full picture of how the business makes money. Old Mission has earned a reputation as one of the most intellectually rigorous and mathematically demanding trading firms, with an interview process that reflects these values.

Old Mission Capital's culture is defined by its small size, intellectual intensity, and close-knit team dynamics. With a relatively compact headcount, Old Mission operates more like an elite squadron than a large corporation — every person on the team is expected to be exceptional, and the close quarters mean that collaboration and interpersonal fit matter enormously. The firm deliberately stays small to maintain this culture and the quality of its team.

The intellectual environment at Old Mission is extremely rigorous. The firm attracts people who are genuinely passionate about probability, mathematics, and the challenge of making better decisions under uncertainty. Conversations on the trading floor often revolve around interesting problems, optimal strategies for games and puzzles, and the mathematical properties of the markets being traded. New hires are immersed in this environment from day one, with experienced traders serving as mentors who challenge them to think more clearly and precisely about risk and reward.

Despite the intensity, employees describe Old Mission as a genuinely enjoyable place to work. The small team size creates a family-like atmosphere where everyone knows each other well, social events are frequent, and there's a shared sense of purpose and pride in the firm's success. The flat organizational structure means that junior traders have direct access to senior leaders and can contribute to strategic decisions early in their careers. Work-life balance is reasonable for the industry — the firm values sustainable performance over burnout — and compensation is highly competitive, with significant profit-sharing that aligns individual incentives with firm success. Old Mission's culture self-selects for people who are intellectually curious, competitive but collaborative, and committed to continuous improvement.

Old Mission Capital's hiring bar is exceptionally high, reflecting the firm's philosophy that a small team of extraordinary individuals outperforms a large team of merely good ones. For trading roles, the firm looks for candidates with outstanding probability intuition, mental math speed, and the ability to think clearly under pressure. The ideal candidate doesn't just know probability theory — they live it, constantly thinking about decisions in terms of expected value, edge, and optimal strategy.

The firm places enormous weight on raw quantitative aptitude. Candidates who have excelled in math competitions (USAMO, Putnam, IMO), programming competitions, or poker/bridge at a high level have a natural advantage because these activities develop the rapid probabilistic thinking that Old Mission values. However, competition pedigree is not strictly required — the firm also evaluates candidates through its rigorous interview process, which is designed to identify people with exceptional quantitative reasoning regardless of their specific background.

Beyond pure math ability, Old Mission looks for candidates who demonstrate composure under pressure, quick decision-making, and strong interpersonal skills. Trading is inherently stressful — you're constantly making decisions with real financial consequences under time pressure and uncertainty. Old Mission wants people who stay calm when things go wrong, who can adjust their thinking based on new information, and who communicate clearly with their teammates. Cultural fit matters enormously at a firm this small — the team needs to work together smoothly every day, so arrogance, inability to take feedback, or poor teamwork are immediate disqualifiers regardless of quantitative talent.

Old Mission Capital's interview process is known for being mathematically intense and highly selective, typically consisting of 3 to 5 rounds over 3 to 6 weeks. The process is heavily weighted toward probability, mental math, and trading-relevant quantitative reasoning. Old Mission's interviews have a reputation for being among the most challenging at any trading firm — they test not just whether you can solve problems but how quickly, accurately, and calmly you work under pressure.

The general structure is as follows:

• Mental math and probability assessment (1 round): An initial timed test that evaluates arithmetic speed and basic probability knowledge. This is a hard filter — if you can't perform mental calculations quickly and accurately, you won't advance. The test is similar to Optiver's 80-in-8 in spirit: rapid-fire arithmetic under severe time pressure.
• Phone screen (1-2 rounds): Technical conversations with traders focusing on probability problems, brainteasers, and sometimes market-making scenarios. These 30-60 minute calls test your ability to think quantitatively in real time while explaining your reasoning clearly. Problems often require creative approaches and careful setup rather than just formula application.
• On-site interviews (2-3 rounds): In-person interviews at Old Mission's Chicago office, typically lasting a half-day or full day. Each round involves progressively harder probability and trading problems, often with a market-making simulation or game-theory exercise. Interviewers will push you to your limits and observe how you handle difficulty, ambiguity, and mistakes.

Throughout the process, Old Mission is evaluating your speed, accuracy, composure, and intellectual depth. They want to see that you can solve problems quickly, but they also want to see that you approach problems methodically, check your work, and remain calm when a question is harder than expected. The interview is designed to simulate the mental demands of actual trading — rapid decision-making under uncertainty with real consequences.

Each stage of the Old Mission interview tests competencies that directly relate to success on the trading floor. Here is a detailed breakdown:

Mental Math: Old Mission tests mental arithmetic rigorously. Expect rapid-fire addition, subtraction, multiplication, and division — sometimes under extreme time pressure (e.g., 80 problems in 5-8 minutes). You'll also face fraction-to-decimal conversions, percentage calculations, and multi-step computations that require holding intermediate results in memory. Speed is critical — Old Mission traders compute prices and risk in their heads constantly throughout the day. Practice daily with Zetamac and aim for scores significantly above average.

Probability and Expected Value: The core of Old Mission's interviews. Questions range from classic problems (dice games, card draws, random walks) to more novel scenarios that require creative thinking. Expect conditional probability, Bayes' theorem applications, expected value calculations with complex payoff structures, and problems involving optimal stopping or strategy selection. Old Mission's probability questions tend to be multi-step and require careful setup — rushing through without thinking clearly is a common failure mode.

Market Making and Trading Games: You'll likely face at least one simulated trading exercise. Old Mission may have you make markets on events (sports outcomes, coin flip sequences), trade in a card game where information is gradually revealed, or quote prices on a simple financial instrument. Key skills: setting appropriate spreads, updating beliefs based on observed trades, managing inventory risk, and staying disciplined even when the game pressures you to take bad risks. The simulation tests both quantitative ability and behavioral composure.

Brainteasers and Puzzles: Old Mission values creative problem-solving and includes brainteasers that test your ability to find clever approaches to unusual problems. These might be combinatorial puzzles, optimization problems, or logical reasoning challenges. The key is to think before computing — often the elegant solution requires a moment of insight rather than brute-force calculation. Talk through your thinking process and be open to hints from the interviewer.

Cultural Fit: Because Old Mission is small, cultural fit is weighted heavily. Interviewers assess whether you'd be a good teammate: Are you pleasant to work with? Do you handle being wrong gracefully? Can you explain your thinking clearly without being condescending? Do you show genuine intellectual curiosity? Being relaxed, authentic, and engaged in the conversation — rather than stiff or performative — signals the kind of person Old Mission wants on their team.

Probability is the foundation of everything at Old Mission Capital. Your preparation should focus on developing the kind of deep, fluent probabilistic thinking that allows you to solve novel problems quickly — not just apply memorized formulas to familiar patterns.

Start with "A First Course in Probability" by Sheldon Ross or "Introduction to Probability" by Blitzstein and Hwang to ensure your foundations are solid. Then move to problem-focused resources: "Fifty Challenging Problems in Probability" by Mosteller, "Problems in Probability" by Shiryaev, and the probability sections of "A Practical Guide to Quantitative Finance Interviews" (the green book). Solve at least 200 problems across these resources, focusing on conditional probability, Bayes' theorem, expected value with complex payoffs, and optimal strategy problems.

For Old Mission specifically, practice problems that involve sequential decision-making and information revelation — scenarios where you observe outcomes and must update your beliefs and actions. These mirror real trading situations where order flow and price movements carry information that you must interpret and act on quickly. Also practice estimation and Fermi problems, which test your ability to decompose complex questions and reason approximately when exact calculations aren't feasible. Time yourself on every problem — Old Mission values speed alongside correctness, so practice solving problems in under 3-5 minutes rather than spending 20 minutes on a perfect solution.

Mental math is a hard gate at Old Mission — if you can't pass the timed arithmetic test, your other abilities won't matter. This means dedicated daily practice is essential, ideally starting months before your interview.

Use Zetamac as your primary training tool. Start with all operations enabled and practice for at least 15-20 minutes daily. Track your scores over time and aim for consistent improvement. Target benchmarks: 50+ correct answers in 2 minutes is a minimum; 65+ puts you in competitive territory for Old Mission. Once you plateau, practice specific weak areas — many people are slower at division or subtraction than multiplication.

Beyond basic arithmetic, practice the specific types of mental calculations that arise in trading: computing percentages of large numbers, converting between fractions and decimals (especially eighths and sixteenths, which appear in bond pricing), estimating square roots, and performing multi-step calculations where you hold intermediate results in working memory. Also practice under stress — have a friend time you, or practice immediately after exercise when your heart rate is elevated. The goal is to make mental math so automatic that interview stress doesn't degrade your performance. Remember that consistency matters more than peak performance — Old Mission wants traders who compute accurately all day, not just during the first 5 minutes.

Old Mission's interviews frequently include interactive trading exercises that are impossible to prepare for through textbook study alone. You need hands-on practice with the dynamics of making markets, updating beliefs, and managing risk in real time.

Start by playing probability-based games with friends: Liar's Dice, poker, or custom market-making games where one person has information and the other must quote prices. These games develop your ability to reason about hidden information, set appropriate risk levels, and make decisions quickly. For more structured practice, set up market-making simulations: one person draws cards or rolls dice and you must continuously quote a bid-ask spread for the final outcome. Practice adjusting your quotes based on observed outcomes and managing your exposure.

Key principles to internalize before your Old Mission interviews: (1) Your spread should reflect your uncertainty — wider when you know less, tighter when you're confident. (2) Trades against you carry information — if someone is willing to trade at your offer price, the true value is likely higher than your midpoint estimate. (3) Inventory management matters — as your position grows, adjust your quotes to incentivize trades that reduce your exposure. (4) Stay disciplined — don't make negative-EV trades just because you're losing. Practice these principles until they're second nature, and you'll perform well in Old Mission's simulations regardless of the specific game format.

For the most realistic Old Mission preparation, Quant Blueprint's coaching program pairs you with mentors from top market-making firms who run live trading simulations and probability speed rounds at the intensity Old Mission expects. Our team of 10 quant traders and researchers understand the specific combination of mental math, probabilistic reasoning, and composure under pressure that Old Mission evaluates — and can pinpoint exactly where you need to improve before interview day.