Ok, a OnePlus Nord it is, for anyone with the Nord wanting to pitch in 
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That sounds like way too much indeed, even if in general after an update battery use can go up for a little while for some OS adjustments.
Since it isn’t mentioned … did you try a reboot already?
In any case, you should have a backup of all your important stuff or be prepared to set everything up again from scratch, just to be cautious …
You can always install the OS from scratch. For the OnePlus Nord, dev and stable releases are available for download, so make sure which one you would want … https://doc.e.foundation/build-status … and which one you’re currently on … Settings - About phone - Android version (tap on it) - /e/ version.
Else, pending any detailed Nord insight coming from somebody else (I don’t have the device), it seems the Nord is a so-called A/B device (slots a and b are mentioned). What is true for other A/B devices might apply here, too …
The OS version before the last update should be available on the now inactive OS slot of the two OS slots. Be prepared to do a factory reset (may or may not be necessary).