We calculate the Ohtsuki invariants λi(M)λi(M)(i=0,1,2,3)(i=0,1,2,3) of every Brieskorn–Hamm manifold MM which is a rational homology 33-sphere. We denote the order of H1(M;ℤ) by H. By the result, we show that the third Ohtsuki invariant λ3(M) of the Brieskorn–Hamm manifolds M with H=1 is negative, and the third Ohtsuki invariant λ3(M) of most Brieskorn–Hamm manifolds M with H≥2 is positive.