I don't know what The Master Cleanse is, never heard of it, but is seems crazy that you "have to stick exactly with the program" to get a result in my opinion, any program (including any recommendations I make with my program) should be flexible enough to incorporate some changes.
Oregano oil can aggravate, but anything can aggravate at anytime, even a probiotic. It's all about test and measure. The "go low and go slow" method always makes sense if you are a sensitive person, regardless of what you take.