I dont think prepooing with oil will block out the benefits of the dc. i do something similar to that in my reggi, i do a hot oil treatment before i poo and then i shampoo and dc, (and yes i still shampoo i know alot of naturals are against it but i find it cleanses my hair perfectly for the deep condition!) but it all depends on what type of oil you use in ur prepoo i think, i use tea tree, peppermint, and olive oil in my hot oil mix and it does wonders in my hair! maybe i am wrong for doin prepooing with oil before i dc, but i find it better this way because i dont like having oily hair, and this way i rinse out the oil, but not completely, so the hot oil wont go to waste you know?