The O2 table: training hypoxia tolerance without crossing the line

Once CO2 tables have done their job, something changes. Contractions no longer frighten you, the first half of a hold feels routine, and you start spending real time in the struggle phase. At that point a new question appears: how does your body, and your mind, cope when oxygen itself starts to run low?

That is the territory of the O2 table. It is a valuable tool, but it is also the one most often misused, sometimes with holds that end above the athlete's personal best. This guide explains what an O2 table really trains, why it deserves more caution than a CO2 table, and how to build one that challenges you without putting you near the edge.

What an O2 table actually trains

During a static hold, two things happen at once: CO₂ rises from the very first second, and oxygen falls. For a long time the oxygen drop is gentle, because your lungs and blood hold a sizeable reserve. Later in the hold the decline speeds up¹. CO2 tables work mostly on the first part of that story, the rising CO₂ and the discomfort it creates. An O2 table deliberately takes you into the second part.

In practice, an O2 table trains three things:

• Familiarity with late-hold sensations. Holds that reach 70–85% of your best put you in the struggle phase for a meaningful time, round after round. You learn to stay relaxed and economical when contractions are strong and frequent.
• Tolerance to lower oxygen levels. Elite apneists finish maximal holds with levels of arterial oxygen that would be striking in untrained people². Hypoxia tolerance is one of the factors that appear to separate strong static performers³.
• Repeated exposure to the diving response. Breath-holding triggers an oxygen-conserving reflex: the heart slows and blood flow is redirected towards the brain and heart⁴. Long holds give that response time to develop.

A note of honesty: research has described what distinguishes good breath-hold divers, but there is little controlled evidence on O2 tables as such. Much of their value is practical: structured, repeated experience of long holds, at a dose you choose in advance rather than one your ego chooses on the day.

Why O2 tables carry more blackout risk

In a CO2 table, the hold stays at around half your personal best. You feel strong discomfort by the end, but your oxygen level remains far from critical. An O2 table is different by design: each hold is longer than the last, and the final ones come close to the zone where oxygen falls quickly.

The problem is that low oxygen gives poor warnings. The urge to breathe is driven mainly by CO₂, not by oxygen, and loss of consciousness from hypoxia can arrive without any reliable signal beforehand¹. Three features of an O2 table make that more likely than in a CO2 table:

• The holds get close to your limit. That is the point of the exercise, which is why the ceiling matters so much.
• Fatigue accumulates. Eight long holds take their toll, and your limit on round 8 is not the same as on a fresh first attempt.
• The table creates a goal. "Finishing the table" can push you to ignore a warning sign you would respect on a single hold.

An O2 table should take you towards your limit, never to it. The ceiling is part of the method, not a sign of timidity.

Before you start: the prerequisites

An O2 table is not a starting point. Tick every box below before you add one to your week.

• Several weeks of regular CO2 tables, typically four to six, with sessions you complete in control.
• A recent, reliable personal best, measured dry and lying down in good conditions. Every number in the table is derived from it.
• Calm in the struggle phase. You can let contractions roll through without tensing up or panicking.
• A safe set-up: lying on a bed or mat, nothing to fall from, nobody depending on you, phone silenced except for your timer.
• No medical red flags. With a heart or lung condition, epilepsy or a pregnancy, talk to a doctor before any breath-hold training.

Build your O2 table safely

The structure is simple. What matters is respecting the limits at both ends.

1. Fixed recovery: 2:00 for every roundLong enough for your breathing to restore oxygen and clear much of the CO₂, so each hold tests your oxygen tolerance rather than leftover CO₂. Never shorten it.
2. First hold: 30 to 40% of your personal bestWith a 3:00 best, start at about 1:00. The first rounds are a warm-up that also triggers the diving response.
3. Progression: +10 to 15 seconds per roundSmall steps keep the curve smooth. Alternating +15 and +10 works well.
4. Last hold: never above 80 to 85% of your personal bestWith a 3:00 best, the final hold is 2:30 at most. If your steps would take you beyond that, use smaller steps.
5. 8 roundsA session takes around 30 minutes. Start with 6 rounds, stopping at a lower final hold, if you are new to the format.

Running the session

Treat an O2 table with the same calm routine as a CO2 table, with one extra rule: the decision to stop is always yours, and it is always the right one.

1. Lie down, dry, well restedNot after a heavy meal, not at the end of an exhausting day. A bed, sofa or mat in a quiet room.
2. Breathe calmly during each recoverySlow belly breathing with a long exhale. No fast, deep breathing to "load up": it removes CO₂ without adding useful oxygen⁵.
3. One full but unforced inhaleAround 85–90% of your capacity. Forcing creates tension, and tension costs oxygen.
4. Relax actively during the holdScan your body from jaw to feet, and let contractions pass like waves. In the late rounds, stay especially still.
5. Recovery breaths at the end of every holdA small exhale, a quick deep inhale, a brief pause under gentle pressure, two or three times. Stay lying down.

How to read your session

The clock is not the only result. What you feel, and when, tells you whether the table is set right and whether you are adapting.

One session a week is enough

The stress of an O2 table is high, and its benefit comes from quality, not volume. For most freedivers, one O2 table a week alongside one or two CO2 tables is the right balance. Keep it away from your personal best attempts: at least 48 hours between the two.

If your goal is a specific milestone, such as breaking 3 minutes, the O2 table is a supporting tool, not the main event. And if you train without a coach, our guide to progressing on your own shows how to fit it into a sustainable routine. For a direct comparison of the two formats, see CO2 table vs O2 table.

Common mistakes

• Ending above your personal best. Some older guides suggest a final hold at or above your best. That turns a training table into a series of maximal attempts in a fatigued state. Stay at 80–85%.
• Shortening the recovery. It mixes two stimuli and sends you into long holds without having recovered. The 2:00 recovery is fixed.
• Hyperventilating between holds. Lowering CO₂ delays the urge to breathe without adding meaningful oxygen, which is how blackouts happen⁵.
• Starting too early. Without a solid base of CO2 work, the late rounds become a fight, and fights waste oxygen.
• Doing it too often. Two or three O2 tables a week accumulate fatigue and erode your margin.
• Finishing the table at all costs. Stopping early on a bad day is a sign of good judgement, not weakness.

The non-negotiable safety rules

• Always dry and lying down when you train alone. Never in the bath, standing up, driving, or anywhere a loss of consciousness could hurt you.
• Never hyperventilate before or between holds.
• Stop the session at the first sign of tingling in the hands or lips, visual changes, confusion, involuntary movements or any unusual sensation. Do not attempt another hold that day.
• Never train when tired, ill or after alcohol. Your limit on those days is lower than your numbers suggest.
• Get medical advice if you have a heart or lung condition, epilepsy, or are pregnant.

Frequently asked questions