If this possible without root? I read somewhere that OREO comes with new modified bootloader that doesn't allow to go back ? Anybody can offer a link to follow instructions?
And here is the baseband version that was on my phone prior to the update. I also downloaded it from Sam Mobile and tried to flash it with Odin but it failed.
Root has nothing to do with downgrading.
Your oreo bootloader version is 4, the nougat version you're trying to flash is 3. You can't downgrade.
And you won't find a v4 nougat firmware.