Not exactly. The opposite of natural law is nihilism, that there is no law. The consequence of wholly accepting that shadow is a choice with no rational basis for either pole: either one wills or one wills not. Affirm life or deny it--but there's nothing left to base an affirmation of life upon but the simply biological will that seeks to escape death, avoid pain and feel pleasure. Thus we come to Hobbes and Mills. Logic (teleologically meaningless but useful as a tool of the will) will tell you that men, being relatively similar in strength, require numbers to protect themselves and to gain any significant power. To do so, we create the Leviathan and invest it with an illusory authority--for if there's no Law, there's no reason to keep one's word when it is not in one's interest, though it may be in one's interest to fool others into thinking illusion real. Thus we have positivism--not the opposite of Law per se, but the consequence of the opposite.
Of course, once the Leviathan exists, what guides it? Certainly not moral laws, save as it needs them to continue to manipulate it's subjects.