You downloaded OSCAR, opened it up, and now you're staring at a wall of colourful squiggly lines wondering what any of it means. You're not alone — almost every new CPAP user goes through this. OSCAR is one of the most powerful tools available for understanding your PAP therapy data, but it hands you the raw detail with very little explanation.
This guide walks through how to read OSCAR CPAP charts panel by panel, in plain English, so you can start making sense of what your machine recorded last night.
Medical disclaimer
The information in this article is for educational and informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss changes to your therapy with your prescribing clinician before acting on anything you see in your data.
What OSCAR Is (and Why PAP Users Use It)
OSCAR — Open Source CPAP Analysis Reporter — is a free, open-source desktop application that reads the detailed data stored on your CPAP or BiPAP machine's SD card. Your machine's built-in app often shows you a single nightly AHI number. OSCAR shows you everything underneath that number: every breathing event, every pressure change, every significant leak, minute by minute across the whole night.
That granularity matters. Two nights can have the same AHI but completely different stories. One might be a handful of positional obstructive events in the first REM cycle; the other might be scattered flow limitations and RERAs throughout the whole night. You can only see that difference when you read the charts.
The Main OSCAR Panels
When you open a daily summary in OSCAR, you'll see several chart panels stacked vertically. Each panel shares the same horizontal time axis — the night runs left to right. Here are the most important ones:
1. AHI / Events
This panel shows your apnea-hypopnea events plotted against time. Each dot or bar represents a detected breathing interruption. OSCAR colour-codes events by type:
OA (Obstructive Apnea)
A full stop in airflow caused by the airway physically collapsing. Usually green or teal.
H (Hypopnea)
A partial reduction in airflow — breathing is happening, but significantly reduced. Usually yellow.
CA (Central Apnea)
A pause in breathing where the airway is open but the brain momentarily stops sending the signal to breathe. Usually blue. Central events during CPAP therapy can sometimes be treatment-emergent, which is worth discussing with your clinician.
FL (Flow Limitation)
Not an apnea, but the airway is partially narrowed, creating a flattened-top shape in the flow waveform. Usually shown in a pink/salmon colour. Your clinician can interpret flow limitation counts in the context of your specific prescription.
RERA (Respiratory Effort-Related Arousal)
A sequence of flow limitations that ends in a micro-arousal — your brain waking you enough to restore airflow without fully waking you. RERAs don't count toward the official AHI but can significantly fragment sleep quality.
Looking at the distribution of events across the night is as important as the total count. A cluster of OAs in the early morning sometimes correlates with sleeping position. Different event distributions across the night represent different data patterns.
2. Pressure
The pressure panel shows the pressure your machine delivered in cm H₂O over the night. If you're on APAP (automatic CPAP), you'll see the pressure trace rising and falling as the machine responds to your breathing. Fixed-pressure CPAP will show a flat line. BiPAP users will see two lines — IPAP (inhale pressure) and EPAP (exhale pressure).
Things to notice:
Pressure ceiling
If your machine is frequently reaching its maximum allowed pressure setting, that pattern is visible in OSCAR's pressure trace.
Pressure hunting
Rapid, erratic pressure fluctuations are sometimes associated with leak artefact in the data.
Correlation with events
Check whether your events cluster at moments of lower pressure or just after a pressure spike.
3. Leak Rate
The leak panel shows total mask leak in litres per minute. A small amount of intentional leak is by design — masks have vent ports that continuously exhaust CO₂. What you're watching for is unintentional leakabove your mask's design threshold.
High leak (typically above 24 L/min for most masks, but check your specific mask specs) means some of the pressure you're supposed to be getting isn't reaching your airway. It can also cause your APAP machine to misread your breathing and respond poorly. Many unexplained AHI spikes trace back to positional leak — rolling onto your side and creating a gap at the mask seal.
Look for:
Sustained high leak
Consistently elevated leak throughout the night suggests a fit issue — mask size, headgear adjustment, or a worn-out cushion.
Intermittent spikes
Brief leak spikes often correspond to swallowing, changing position, or an event.
Large mouth leak
If you use a nasal or nasal pillow mask and breathe through your mouth during the night, that shows up as sustained high leak.
4. Flow Rate
The flow rate panel is the raw breathing waveform — it shows actual airflow in and out of your lungs throughout the night. This is the richest panel in OSCAR, and also the most detailed.
Each breath appears as a wave: inhalation peaks upward, exhalation dips downward. On a healthy breath with adequate pressure, the inhalation has a smooth, rounded peak. Flow limitation appears as a flattened top on the inhalation — the curve is cut short before it reaches a natural peak, a pattern the machine flags as flow limitation.
Zooming in with OSCAR's time-range selector (drag to zoom on the time axis) lets you inspect individual breath shapes. This is where reading OSCAR really pays off — you can see the subtle deterioration in breath shape in the minutes before an event, or confirm that an apparent event flag was actually a swallow artefact.
5. SpO₂ and Pulse (if available)
If your machine has a pulse oximeter or you've connected one, OSCAR plots your blood oxygen saturation (SpO₂) and pulse rate. These panels are informational context, not diagnostic tools. Large drops in SpO₂ that correlate with event clusters are the kind of thing worth noting for your next clinic appointment.
How AirwayLab Shows the Same Data — in Your Browser
If the OSCAR installation process put you off, or if you want a second way to look at your data, AirwayLab reads the same SD card files directly in your browser. No software to install, and your data never leaves your browser— it's processed locally, in the same tab.
AirwayLab presents your AHI breakdown, flow limitation density, RERA index, leak statistics, and pressure summary in a single-page view alongside trend charts across multiple nights. The flow waveform viewer lets you zoom into individual breathing sequences the same way OSCAR does.
The two tools are genuinely complementary. OSCAR gives you fine-grained per-breath inspection and a desktop environment that's hard to beat for deep dives. AirwayLab gives you an instant overview without installation friction, makes multi-night trends easy to scan, and works on any device — including the laptop you bring to a clinic appointment.
A Practical Reading Workflow
When you open a night in OSCAR (or in AirwayLab), try this sequence:
Start with the AHI panel
What's the total? Where are events clustered? Any pattern by time of night?
Check leak rate
Rule out a leak-driven false event count before digging into event types.
Look at the pressure trace
Was the machine working hard? Did it hit its ceiling?
Zoom into event clusters
Switch to the flow rate panel and zoom into a 5–10 minute window around a cluster. What does the breath shape look like in the lead-up?
Note FL and RERA patterns
Even if your AHI looks reasonable, high flow limitation or RERA counts are additional data points beyond the AHI number.
Bring a screenshot or AirwayLab summary to your clinician
Your data supports the conversation — your clinician makes the call.
When to Talk to Your Clinician
Reading your OSCAR charts is about understanding your data, not about treating yourself. If you're seeing:
Consistently elevated AHI across multiple nights
High counts of central apneas
Flow limitation or RERA counts that remain elevated across multiple nights
Unexplained SpO₂ dips
...those are things to bring to your prescribing clinician or sleep technologist, not things to self-adjust around. Share your data — both OSCAR screenshots and an AirwayLab summary are easy to attach to a patient portal message.
Read Your CPAP Data in Your Browser
Upload your SD card data to AirwayLab and see your breathing patterns the same way OSCAR shows them — free, in your browser, and with your data staying on your device.
Related reading
How to Read Your CPAP Data — why AHI isn't the whole story and what metrics to look at next.
AirwayLab vs OSCAR — what each tool does best and how to use both together.
What Is Flow Limitation on CPAP? — understanding the flattened waveform and what it means.