Which stock rom do you want? Frequently question about this topic are answerd here https://xdaforums.com/oneplus-5/how-to/official-oxygenos-4-5-2-7-1-1-ota-t3627003 you should start from 5.1.4.hi, my op5 is running 8.0.0 using xXx.
im forced to go back to stock, is there any special consideration i should take into account before flashing? i dont want to brick my phone, cheers