What is your evidence for this, Fejal? What studies have been done? Which oils in particular are you referring to? Toxic in what sense?
I don't know the specifics for oregano oil but one animal research article only listed it for external use so I tend to think that it's a good idea to assume toxicity in light of the concentration. Given that there are proven drugs to kill the bug it just doesn't make sense to use it. I can see people who do as desperate due to being undiagnosed and not under the care of a physician. I doubt any MD would prescribe it internally.