[en] We present a search for B_S⁰ oscillations using semileptonic B_S → D_sμX (D_S → K_S⁰K). The data were collected using the D0 detector from events produced in √s = 1.96 TeV proton-antiproton collisions at the Fermilab Tevatron. The Tevatron is currently the only place in the world that produces B_S⁰ mesons and will be until early 2008 when the Large Hadron Collider begins operating at CERN. One of the vital ingredients for the search for B s oscillations is the determination of the flavor of the B_S⁰ candidate (B_S⁰ or (bar B)_S⁰ ) at the time of its production, called initial state flavor tagging. We develop an likelihood based initial state flavor tagger that uses objects on the side of the event opposite to the reconstructed B meson candidate. To improve the performance of this flavor tagger, we have made it multidimensional so that it takes correlations between discriminants into account. This tagging is then certified by applying it to sample of semimuonic B^(0,+) decays and measuring the well-known oscillation frequency (delta)m_d. We obtain (delta)m_d = 0.486 ± 0.021 ps^-1, consistent with the world average. The tagging performance is characterized by the effective efficiency, εD² = (1.90 ± 0.41)%. We then turn to the search for B_S⁰ oscillations in the above-named channel. A special two-dimensional mass fitting procedure is developed to separate kinematic reflections from signal events. Using this mass fitting procedure in an unbinned likelihood framework, we obtain a 95% C.L. of Δm_s > 1.10 ps^-1 and a sensitivity of 1.92 ps-1. This result is combined with other analyzed B_S⁰ decay channels at D0 to obtain a combined 95% C.L. of Δm_s > 14.9 ps-1 and a sensitivity of 16.5 ps-1. The corresponding log likelihood scan has a preferred value of Δm_s = 19 ps-1 with a 90% confidence level interval of 17 < Δms < 21 ps-1, assuming Gaussian uncertainties. A comparison of the change in the likelihood between Δm_ss = 19 ps-1 and Δm_s = ∞ yields an 8% expectation for a background fluctuation