Why doesn't Oura write HRV to Apple Health?
Short answer
Oura does not include HRV in the data categories it exports to Apple Health. Your HRV can exist inside Oura while the Apple Health HRV category has no Oura samples. Changing Health permissions cannot enable a category that the Oura integration does not write.
What Oura sends to Apple Health
Oura's current Apple Health integration list includes active energy, heart rate, respiratory rate, sleep, steps, weight, routes, and workouts. HRV is not on that export list.
This is an integration choice, not evidence that your ring failed to record HRV. The Oura app can still use its own HRV data in Oura features without creating an HRV sample in HealthKit.
Why the permission screen does not fix it
Apple Health permissions control which supported categories an app may read or write. On iPhone, the path is Health, your profile picture, Apps, then Oura. If HRV is absent from Oura's supported write categories, there is no HRV switch that can make Oura publish it.
Apple documents the same permission model for apps and devices connected to Health. Permission grants access. It does not change the data types implemented by the app.
Ways to use the Oura HRV data
For a file, request your personal data from the Oura Membership Hub or download selected metrics from Oura on the Web. For an ongoing connection, use a service with a direct Oura integration or the Oura API instead of routing this metric through Apple Health.
Keep Apple Health enabled for the categories Oura does support. Another device can still write HRV to Health, so always check the source name on a Health data sample before comparing it with Oura.
How Gyroscope does it
Gyroscope has a live direct Oura integration. It also reads HealthKit HRV in SDNN form along with sleep, resting heart rate, workouts, and the other supported HealthKit categories.
That keeps the direct Oura source separate from HRV samples written to Apple Health by another device. The merged timeline is deduplicated against your primary device instead of assuming that every source wrote the same categories.