Fredholm conditions and an index formula are obtained for WienerHopf operators W (a) with slowly oscillating matrix symbols a on weighted Lebesgue spaces L p N (R+, w) where 1 < p < ∞, w is a Muckenhoupt weight on R and N ∈ N. The entries of matrix symbols belong to a Banach subalgebra of Fourier multipliers on L p (R, w) that are continuous on R and have, in general, different slowly oscillating asymptotics at ±∞. To define the Banach algebra SOp,w of corresponding slowly oscillating functions, we apply the theory of pseudodifferential and Calderón-Zygmund operators. Established sufficient conditions become a Fredholm criterion in the case of Muckenhoupt weights with equal indices of powerlikeness, and also for Muckenhoupt weights with different indices of powerlikeness under some additional condition on p, w and a.