The battery drain on my Oasis is significantly poorer than on other Kindles I've owned but I've not had that sort of drain in Sleep mode other than when I've had a book stuck in the indexing process. Assuming you've done a full Restart by holding the Power button in for 40 seconds before releasing it I suggest you speak to Customer Service.
A week ago I Reset mine to Factory Defaults and, provided I only load around 10 books at a time and check the indexing every 30 minutes or so (to ensure that none of them is stuck in the indexing process) the battery drain has improved to about where it was when new in 2017. It used to take around 20 minutes to do a Restart but that's improved to no more than a few minutes now.
Mine does the same thing. With Bluetooth and wifi off off and in airplane mode, the light at 4 or 5 my kindle is good for about 3-4 hours of reading. My battery today went from 100% to 51% in about 2 hours. On the upside I plugged it in and it was back up to 100% in less than 15 minutes which is also not normal and indicative of a bad battery. My kindle is out of warranty and I'd be happy to try to replace the battery if I could only find a battery. I certainly can't take the kindle very far with this battery life and when I travel I take my old paperwhite instead of the oasis.