While it's not necessarily unheard of for fitness tracking devices to have this feature, Fitbit seems to have turned it on for some trackers without mentioning it to users.
The quiet rollout looks to be for a few different trackers too. Though it may not be hitting every single tracker that Fitbit sells.
Blood Oxygen tracking is rolling out to the Fitbit Charge 3, Versa, & Ionic
As of right now it seems users are reporting the feature showing up for the Fitbit Charge 3 trackers. At least two people in the Reddit thread note it arriving for their devices.
Engadget also reports that some users have noted it showing up on Fitbit Ionic and Fitbit Versa devices as well. Whether or not Fitbit will officially announce this feature is unclear but if you have one of these trackers, or a recent one, keep an eye out for it.
Updates to the tracker itself may need to be done, and there may need to be an update for the app installed on your phone as well.
Fitbit's aim for adding this feature to some of its trackers is so that users can keep an eye out for more uncommon oxygen variation. Bigger fluctuations can be a sign of respiratory problems like asthma.
And while this data within the app isn't meant to give out any sort of diagnosis, it can still can be a good way to indicate if there's something you should be watching out for.
You don't need Fitbit Premium
While it's not clear if the feature will roll out to more than just the Charge 3, Ionic, and Versa, what is clear is that you don't need Fitbit Premium.
The feature will be available to premium members, but users reporting about the feature state that they don't have a premium membership, which Fitbit charges a monthly fee for.
Users don't seem to be certain when oxygen variation started tracking, but it's unlikely that it's been more than a week. That being said it's always possible that it's been there for a little while and people either never noticed it or just didn't think to mention it to other users.
Either way, it's arriving now and that adds just a little more value to some of Fitbit's best devices.