SpO2 / Blood Oxygen is the smartwatch feature that estimates the percentage of oxygen carried in your blood. It works through photoplethysmography, or PPG: red (around 660nm) and infrared (around 940nm) LEDs on the caseback shine light into your skin, oxygenated and deoxygenated hemoglobin absorb those wavelengths differently, and the watch calculates the ratio from the light that bounces back. The catch is that the wrist relies on reflectance PPG, which is noisier than the transmission oximeter clipped to your fingertip at a clinic. It is easily thrown off by motion, skin tone, tattoos, cold hands and a loose strap, and accuracy falls away once readings dip below roughly 90 percent.
Apple put the feature on the map, adding blood oxygen to the Apple Watch Series 6 in 2020. The Masimo patent fight then forced Apple to sell US models with the sensor disabled in software from January 2024, before an August 2025 workaround re-enabled it by offloading the calculation to a paired iPhone; in November 2025 a jury ordered Apple to pay Masimo $634 million. Garmin takes a quieter route: its Pulse Ox runs on the Fenix 8, Forerunner 965, Venu and Instinct lines for altitude acclimation and overnight sleep, with the Venu 3 around $450. Samsung was first to earn FDA clearance for sleep apnea detection in early 2024, with the Galaxy Watch 7 from about $300 and the Galaxy Watch Ultra at $650. Withings folds SpO2 into a true analog dial on the hybrid ScanWatch 2, which runs roughly 30 days per charge.
Start by deciding what you actually want the number for. Hikers and mountaineers tracking acclimatization are best served by Garmin, whose watches log overnight and on-demand readings; watching a sea-level 95 to 100 percent settle into the 88 to 92 percent band above 2,500 meters genuinely tells you something. If sleep and possible breathing disruptions are the goal, Samsung's apnea screening or Garmin's nightly SpO2 charts are a sensible starting point. If you refuse to give up a proper watch face, the ScanWatch 2 is the obvious pick. The non-negotiable caveat across all of them: these are wellness readings, not a medical diagnosis.
Knowing the limits keeps your expectations honest. During an apnea event oxygen may dip only about 3 percent, so your watch can read normal while your sleep is genuinely disrupted. A single low spot reading is usually bad contact or a cold finger rather than a crisis, and the trend over several nights is far more useful than any one figure. If you do see persistently low numbers that worry you, treat the watch as a prompt to see a doctor and confirm with a clinical oximeter, not as the final word.