How to Read OSCAR CPAP Charts: AHI, Leaks, Flow & Pressure

OSCAR turns the raw data from your CPAP's SD card into visual charts so you can see exactly what happened overnight: your AHI, leaks, airflow, and pressure. Two nights can show the same AHI number but tell completely different stories. OSCAR shows you the story under the number.

This guide walks through every panel in the correct order (Overview, AHI events, leaks, flow, pressure, and oxygen [SpO2]), shows you what good and concerning patterns look like, and provides a pattern-to-fix table you can act on tonight. The typical goal for adults on effective CPAP therapy is an AHI under 5. You can learn more about what your AHI score means in our detailed breakdown.

Controlling leaks is just as important, since large leaks can make your therapy data unreliable and disrupt your sleep. Always work with your clinician before changing any prescribed setting.

1. What Is OSCAR and How Does It Help You Track CPAP Data?

OSCAR (Open Source CPAP Analysis Reporter) is free software that converts the raw data from your machine's SD card into visual charts. It shows everything underneath your single nightly AHI number: every event, every pressure change, every leak, minute by minute. [2]

The software has two main views. The Overview tab summarizes your long-term trends, showing usage and average AHI over weeks or months. The Daily view provides detailed graphs for a single night, including AHI Events, Leak Rate, Flow Rate, and Pressure, all aligned by timestamp.

Here's a quick summary of what OSCAR tracks:

One practical note: OSCAR reads data from the SD card, so you need a working card to get started. ResMed AirSense machines ship with one, but cards can fail or go missing over time. If yours isn't working, replacement SD cards are available.

2. How to Open and Read Your OSCAR Charts Step by Step 

Before interpreting any single metric, set up your charts correctly. Import your data from the SD card, align the time axis across all panels, then read them in order. Reading panels out of sync is the most common beginner error. If you're new to CPAP therapy, our beginner tips guide covers the fundamentals of setup.

Here's the basic process:

1. Import your data. Insert your SD card into the computer and open OSCAR. Select your device profile and choose the Daily View for your full nightly report.

2. Align the time scales. Make sure the time axis across all charts matches. This ensures AHI Events, Leak Rate, Flow Rate, and Pressure are synchronized so you can see what happened at the same moment.

3. Follow the reading order. Start at the Overview tab for averages. Then, in the Daily view, move through AHI Events, Leak Rate, Flow Rate, Pressure, and Oxygen (if imported). This is the order we'll use throughout this guide.

4. Zoom into unusual periods. Focus on any spikes, high leaks, or breathing irregularities. You can zoom into shorter time windows to see exactly when events occurred during the night.

5. Look for repeating patterns. Check multiple nights of data. This helps you determine if a problem is happening consistently before assuming something is wrong with your setup.

Common mistakes to avoid

• Judging your therapy from one single night instead of reviewing patterns over time.

• Ignoring the leak data. Large leaks can degrade event data, such as AHI or pressure, and disturb sleep quality. [5]

• Comparing OSCAR results between different CPAP machine brands without noting that chart layouts or "large leak" definitions might vary.

• Changing your prescribed pressure settings without clinical guidance.

• Misinterpreting short, isolated spikes as consistent therapy issues.

• Judging a single number instead of the distribution across the night. A cluster of events in one REM cycle is a different problem from scattered events all night long.

For more navigation tips, the OSCAR community resource "OSCAR: The Guide" on ApneaBoard provides additional setup visuals and FAQs.

3. OSCAR Event Flags Explained (OA, CA, H, FL, RERA) 

The AHI Events panel color-codes each breathing event by type. Knowing the difference matters: an obstructive apnea and a central apnea look similar on the count but mean very different things. Here's what each flag means. [8]

Quick Note: OSCAR's colors are user-configurable, so the colors above are typical defaults, not fixed. You can confirm or change them in OSCAR's settings menu.

OA, CA, and H events all count toward your AHI. RERA does not, but it still fragments sleep and is worth tracking.

One clinically important distinction: if you notice a high number of CA (clear airway / central) events, that's worth raising with your clinician. Central apneas have different causes and treatments from obstructive apneas. Our obstructive vs. central sleep apnea guide explains the differences in detail.

4. What Does AHI Mean on OSCAR and What Numbers Are Normal? 

The Apnea-Hypopnea Index (AHI) is the average number of apneas and hypopneas per hour of sleep. Most clinicians target an AHI under 5 for adults on effective CPAP therapy. [3] OSCAR shows this metric for every night, so you can track trends over time.  

For a deeper breakdown, see our AHI score guide. Meanwhile, here are the standard clinical classifications:

AHI values often change from night to night, even in well-controlled patients. Small increases (for example, from 1.5 to 4.0) are normal. [1] This variation can result from changes in sleep position, shifts in sleep stages, or nasal congestion. Focus on your multi-night trend in OSCAR rather than worrying about isolated spikes.

Your AHI accuracy can also be affected by mask leaks. A 2025 study in Respiratory Research found that some CPAP devices exclude events during major leak periods, which can make your reported AHI appear lower than the actual number of breathing interruptions. [6] Always interpret your AHI charts alongside your leak rate graphs.

"You might have a super high AHI on your machine, and it is all because you are not putting on the mask correctly. The mask being loose causes leaks, which the machine registers as a change in flow, so it registers it as an event."

5. How Do Mask Leaks Affect AHI Accuracy in OSCAR?

Large leaks do two things: they fragment your sleep, and they make your AHI unreliable. When too much air escapes the circuit, the machine can't distinguish a real event from the leak, so it can undercount or overcount. Controlling leaks comes before any change to the pressure or mask. 

There are two types of leaks to be aware of. Intentional leaks are normal: your mask vents exhale CO2 by design. Unintentional leaks are the problem. These come from a poor mask seal, an open mouth, or worn-out mask components. [5]

By default, in line with ResMed guidance, OSCAR flags additional unintentional leaks above 24 L/min as a large leak. That's the threshold where event detection starts to suffer.

How much does a leak actually matter? A 2023 study in Respiratory Care tested how unintentional leaks degrade auto-CPAP performance. With leaks present, the ResMed AirSense 10 failed to respond to obstructive events. The pressure during an obstructive apnea fell to about 2.5 cmH2O, compared with 15.8 cmH2O without leak. [7] A big leak can hide real events from both the machine and your AHI number.

Common signs that leaks may be affecting your therapy:

•        Dry mouth or throat upon waking

•        Hissing or whistling sounds near the mask or tubing

•        Air blowing toward your eyes

•        Waking up frequently during the night without a clear cause

•        OSCAR Leak Rate chart showing extended peaks well above the baseline

•        Unexplained AHI spikes that trace back to rolling onto your side (positional leak)

Small changes can often minimize leaks before you consider changing the pressure or mask. Our guide to reducing CPAP mask leaks covers practical adjustments for every mask type. Products like mask liners can improve the seal and reduce skin irritation. If mouth breathing is the issue, Hush Strips or CPAP tape can help keep your mouth closed overnight. Proper cleaning with a hose brush set and CPAP wipes prevents facial oils from weakening the mask seal over time. 

6. How to Read Flow Rate Charts and Spot Breathing Issues

The Flow Rate panel is the raw breathing waveform and the richest chart in OSCAR. A healthy breath has a smooth, rounded inhalation peak. A flattened top means flow limitation: the airway is partially narrowed even though no apnea was scored. 

Here's a quick guide to common flow shapes:

Flow limitations may not always show as apnea events, but they can still reduce sleep quality if they're persistent. Minor waveform changes are often caused by nasal congestion, allergies, low humidity, or sleeping on your back. [4]

Before assuming a major therapy issue, try these safe checks:

•        Ensure your nasal passages are clear

•        Verify your humidifier settings are adequate for the room's air

•        Avoid tucking your chin tightly or sleeping with poor neck alignment

If you consistently experience flow limitations, it may be related to your mask fit or nasal interface design. A light, high-seal nasal mask can improve airflow comfort. The ResMed AirFit P10 is ideal for stable, low-resistance breathing, and the F&P Evora Nasal Mask offers a lightweight design that supports consistent airflow.

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Flow limitation often comes with oxygen dips that you can't see unless you import oximetry data into OSCAR, which brings us to the next section.

7. Adding Oxygen (SpO2) Data to OSCAR

OSCAR can do something your machine's app can't: overlay your blood-oxygen (SpO2) against your flow and events on the same timeline. If you want to see whether a cluster of events actually dropped your oxygen, you need a recording pulse oximeter that exports data OSCAR can read. 

AHI counts events, but it doesn't tell you how far your oxygen fell or how long it stayed down. An overnight oximeter fills that gap. It imports into OSCAR as its own panel, time-aligned with your flow chart, so you can see exactly what happened to your oxygen during each event.

What to look for: oxygen desaturations (dips of 3% to 4% or more) that line up with apneas or flow limitation on the chart above. Isolated dips are normal, but repeated deep dips that track your events are worth showing your clinician.

If you want the full picture, a recording oximeter is the tool. The SleepHQ O2 Sleep Tracking Ring Pro records overnight and is built for this kind of data review. For a primer on how pulse oximeters work, see our pulse oximeter explainer, or browse our best pulse oximeters roundup for a comparison of options.

Adding SpO2 data is entirely optional. OSCAR works perfectly well for basic chart reading without it. But for users who want deeper insight into how their breathing events affect oxygen levels, it's a valuable addition.

8. Why Does My CPAP Pressure Keep Changing in OSCAR?

If you use an Auto-CPAP (APAP), the pressure line rises and falls throughout the night because the machine responds to your breathing in real time. Most of these changes are intentional and adaptive, not a fault. Fixed-pressure CPAP shows a flat line. For a full overview, see our CPAP pressure settings guide.

The most common causes of pressure spikes during the night:

• Flow limitations: When the machine senses restricted airflow (flattened peaks on your flow chart), it temporarily increases pressure to clear the obstruction.

• Apnea or hypopnea events: If the machine detects a pause or significant shallowing in your breathing, it raises pressure to reopen the airway.

• Mask leaks: Large or fluctuating leaks can confuse the machine's algorithm. It might misinterpret the air loss as a breathing event and adjust pressure unnecessarily.

• Positional apnea: Rolling onto your back can cause temporary airway narrowing, which may increase pressure. A CPAP-friendly pillow like the Best in Rest Memory Foam CPAP Pillow can help keep your head and neck in a better position.

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• Mouth opening: Breathing through your mouth can change the airflow pattern enough to trigger a pressure adjustment.

If you use a BiPAP machine, you'll see two pressure lines in OSCAR: IPAP (inhale) and EPAP (exhale), not one. Our EPR explainer covers the difference between exhale pressure relief and full bilevel therapy.

If your AHI stays elevated after fixing leaks and adjusting your position, an auto-adjusting CPAP machine can fine-tune pressure automatically throughout the night.

9. What Common Patterns Mean in Your OSCAR Data (Decision Guide)

OSCAR is most useful when you read patterns over several nights, not single anomalies. This table maps what you see to the likely cause and the first safe things to try. Always review persistent issues with your clinician before changing prescribed settings.

Review your data over several nights before drawing conclusions. Never self-adjust prescription pressure. Share your OSCAR reports with your clinician so they can see the same patterns you do.

Frequently Asked Questions

Is OSCAR CPAP software safe to use?

Yes. OSCAR (Open Source CPAP Analysis Reporter) is free, open-source software that only reads the data already recorded on your CPAP's SD card. It does not change your machine settings or send your pressure prescription anywhere. It cannot alter your therapy; it only visualizes the data. As always, discuss any changes to your therapy with your clinician.

What is a good AHI on OSCAR?

Most clinicians target an AHI of fewer than 5 events per hour in adults on effective CPAP therapy. Night-to-night swings are normal. Focus on your multi-night trend in OSCAR rather than a single high night, and always check your leak chart, because a large leak can make your AHI look falsely low.

What do the colors and letters mean on the OSCAR AHI chart?

OSCAR color-codes events by type: OA is an obstructive apnea (airway collapses), H is a hypopnea (partial reduction), CA is a clear-airway or central apnea (airway open, no brain signal), FL is flow limitation (airway partly narrowed), and RERA is a respiratory effort-related arousal. OA, CA, and H count toward your AHI; RERA does not. Colors are configurable, so confirm yours in OSCAR's settings.

What is a large leak in OSCAR, and what number is too high?

OSCAR flags additional unintentional leaks above 24 L/min as a large leak, in line with ResMed guidance. A small intentional leak is normal because mask vents exhaust CO2. A sustained leak above that threshold reduces the machine's accuracy in detecting events and can fragment your sleep, so fix the seal before considering any pressure changes.

Can OSCAR show my blood oxygen (SpO2)?

Yes, if you record it. OSCAR does not read oxygen from your CPAP, but it can import data from a compatible overnight pulse oximeter and overlay your SpO2 against your flow and events on the same timeline. This lets you see whether breathing events actually reduced your oxygen levels. A recording oximeter such as a sleep-tracking ring is what makes this possible.

References

1. Liobard, H., & ResMed. (2025). "Why does my apnea-hypopnea index (AHI) change?" ResMed.

2. OSCAR, Open Source CPAP Analysis Reporter. SleepFiles.

3. Cleveland Clinic. (2025). "Apnea-Hypopnea Index (AHI)."

4. Williams, C., MD. (2022). "Test your CPAP IQ." Mayo Clinic Health System.

5. American Thoracic Society. "Two head scratchers: What is causing the leak?"

6. Vidal, C., Mallet, J., Skinner, S., et al. (2025). "Concerns arising from the calculation of the apnea-hypopnea index during CPAP-telemonitoring of patients with obstructive sleep apnea." Respiratory Research, 26(1), 244.

7. Fasquel, L., Yazdani, P., Zaugg, C., et al. (2023). "Impact of Unintentional Air Leaks on Automatic Positive Airway Pressure Device Performance in Simulated Sleep Apnea Events." Respiratory Care, 68(1), 31-37.

8. Berry, R.B., Budhiraja, R., Gottlieb, D.J., et al. (2012). "Rules for scoring respiratory events in sleep: AASM Manual update." Journal of Clinical Sleep Medicine, 8(5), 597-619.