CO₂ vs O₂ tables is the question every freediver and spearfisher hits once they start training apnea seriously: which one should you actually do? The short answer is that they train two different things, you'll eventually want both, and you must never run them on the same day. This guide lays the two side by side, explains what each genuinely trains, tells you when to prioritise which, shows how to work both into a training block, and gives you a simple rule of thumb to decide.
The Core Difference in One Side-by-Side
Both are structured tables of holds and rests, but they move opposite variables:
| CO₂ table | O₂ table | |
|---|---|---|
| Hold length | Fixed every round | Increases every round |
| Rest length | Shrinks every round | Fixed every round |
| Main stress | High CO₂ (hypercapnia) | Low O₂ (hypoxia) |
| Trains | Tolerance to the urge to breathe | Working on low oxygen |
| Risk level | Lower — can be done dry and solo | Higher — needs a spotter |
| Best for | Comfort, calm, reef bottom time | Extending a maximal hold, depth |
For the full mechanics of each, see CO₂ Tables for Freediving and O₂ Tables for Freediving. This post is about choosing between them. Both sit inside the system in Breath-Hold Tables: The Complete CO₂ & O₂ Guide.
What Each Table Actually Trains
A CO₂ table trains you to deal with the burning feeling of needing to breathe. The urge to breathe is driven by carbon dioxide, not oxygen, so raising your CO₂ tolerance makes the whole dive feel calmer and the contractions arrive later. This is mostly a mental and diaphragmatic adaptation, and it's the one that most improves everyday spearfishing comfort.
An O₂ table trains your body to operate when oxygen is genuinely low. The adaptations are more physiological — a stronger dive reflex and better tolerance to hypoxia — and they show up as a longer maximal hold. That matters most for deep diving and for divers chasing a personal best.
When to Use Which
A practical way to choose:
- New to tables, or a shallow-reef spearo? Start with CO₂ tables. They're lower-risk, can be done dry and solo, and target the comfort that helps most.
- Have a stable max hold and a reliable spotter? Add O₂ tables to push your ceiling, especially if you dive deep.
- Training for a specific goal? Bias toward CO₂ for calm and endurance on repetitive dives; bias toward O₂ for a longer single maximal hold.
- Short on training time? Default to CO₂ work — it gives most spearfishers the better return per session and carries less risk.
Why You Never Do Both the Same Day
This is the one hard rule. CO₂ and O₂ tables each impose a serious, distinct stress, and stacking them in a single session means you train neither well and recover from neither properly — while compounding blackout risk. Separate them by at least a day.
Working Both Into a Training Block
Once you're ready to train both, alternate them across the week and let CO₂ work carry the larger share. A balanced week might be:
| Day | Session |
|---|---|
| Monday | CO₂ table (dry, solo OK) |
| Tuesday | Rest |
| Wednesday | O₂ table (with a spotter) |
| Thursday | Rest |
| Friday | CO₂ table |
| Weekend | In the water, or off |
That's two CO₂ sessions to one O₂ session — a sensible ratio for most spearfishers. The full programming logic, recovery, and overtraining signs are in how often to do CO₂ and O₂ tables.
A Simple Rule of Thumb
If you remember one thing: train CO₂ for comfort, O₂ for capacity — and never both at once. Start with CO₂, earn a stable max hold, then layer in O₂ tables with a spotter when you want to extend your ceiling. Do that, and the "CO₂ vs O₂" question answers itself over the course of a training block.
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