So, anyone who has nougat december build, if you meet the following conditions, you should have zero trouble installing oreo:
1. You do not have twrp installed permanently
2. You installed magisk by either flashing the patched_boot.img which magisk manager creates OR have installed magisk by booting twrp temporarily.
3. You do not use supersu or flashfire
Now go ahead in the sequence if you meet the above conditions and I can guarantee you will have zero trouble with the tool.
1. Download and install the version 1.2 of the update engine from magisk manager and reboot.
2. Once the phone is rebooted, open update engine from app drawer and press start. Depending upon your internet speed, you will get a confirmation of installation successfull status sooner or later. You will see a progressbar during the process.
3. Dont reboot the device yet, Just go to magisk manager and press install, select install on second slot. Once the process completes, reboot the phone and you will be up and running oreo. Magisk will be preserved.
4. Delete the 1.2 update engine module from magisk manager app and install 1.3 update manager. Reboot the device
5. Repeat the step 3 and 4 for the latest OTA.