The issue with mixing the oil with conditioner is that conditioner absorbs water (due to the fact they contain humectants). This isn't ideal because the purpose of pre-poo is to prevent water from penetrating the cortex and medulla of your hair strand and damaging the cuticle. So, it's best to do oil first, let that penetrate your cuticle and then use conditioner at the very end to detangle. If you mix with conditioner, then your cuticle may swell once you start washing. This problem is more of an issue for high porosity or medium porosity hair.