Android O contains plenty of power-saving improvements for the platform. But it would appear that at least one of the changes won’t be improving your battery life. For some users, on updating to Android Oreo (8.0 or 8.1), “mobile data always active” in Developer options will be enabled. While that will make switching between Wi-Fi and mobile data faster, it may also consume a bit more power.
Our own Artem noticed it enabled on his device although he hadn’t flipped that switch himself.
[FYI] For some, mobile data is now set to “always active” by default in Android Oreo was written by the awesome team at Android Police.