Hi guy's so i was able to downgrade from Pie ( One Ui ) to Oreo
( save your media on your sd card or your PC then you check if they are there )
1 - Go to TWRP or Recovery to wipe data
2- Download and unzip
https://drive.google.com/file/d/1FQKt_lWUxxbaZGk37r4tkEMO0Faaci-R/view
and
https://drive.google.com/file/d/1-0sJtCjGrlmS4z_dDO_FXozR1g8u-vm8/view?usp=sharing
Put RMM STATE BYPASSER AND VERITY IN A SD CARD
3- Open Odin and untick auto reboot
4-click start on Odin now it's fully flash and now press power + volume down button and after the screen turn off directly press Power + volume down and Up now you are in download mode and check if the auto reboot is unticked
5- Click on reset on Odin and now on AP put TWRP and click on start now press Power + volume down and after the screen turn off press Power + Volume Up and now you are on TWRP go on wipe and click on format data and now go back and go to install and click on up a level and got extsd and now you flash RMM state bypasser and after verity and now reboot now you are on oreo and you install magisk after first reboot.