This is not a guide for the android development savy users but for people who preferred oreo to pie. It works without an unlocked bootloader and a rooted phone. You will have to wipe your data so backup everything beforehand!
1) First download the zip that includes Oreo with the November security patch and put it in the root of the SD card.
I have found links for both TA1012 and TA1004 but reportedly they both work for all Nokia 8 phones as the main payload.bin file is the same. I used the TA1012 zip with my TA1012 phone.
Don't worry about the TA-1004 stuff, all Nokia 8 software is generic, only google makes a difference between the model IDs.
Rollback for TA-1012 from Pie Beta to Latest Oreo
Official link just captured a minute ago.
2) Boot into recovery mode.
a) To do this on Pie, plug your phone into a computer and wait for the drivers to get installed. Windows 7 downloaded a few megabytes from windows update for this.
b) Unplug your phone and turn it off. Hold Volume up and plug your phone into your computer. Keeping holding volume up.
c) When the sad android appears with 'No command' showing, release Vol+. Hold the power button and press Vol+ to get into the recovery menu.
3) Use the Vol+ and Vol- to navigate down to 'Apply Update From SD card' click the power button to select it. In the next menu select the downloaded zip file. It'll then verify the downloaded zip and install it. This takes a few minutes.
4) Select Wipe data/factory reset from the recovery menu. You need to do this cause data created on Pie doesn't work on Oreo.
5) Finally select Reboot system now and it'll boot into Oreo. This takes a few minutes.
Hopefully this helps someone. Thanks to @THMSP, @TeddyBeers and @al mak for various posts that helped me downgrade and write this guide.