The adsorption of molecular oxygen (enriched with O-17) onto high surface area MgO has been studied by electron paramagnetic resonance (EPR) spectroscopy. The oxide surface was pretreated in such a way so that surface trapped electron F-S(+)(H) centers are produced. Subsequent dioxygen adsorption results in an electron transfer reaction from F-S(+)(H) centers to O-2, producing a surface stabilized superoxide (O-2(-)) anion. The resulting EPR spectrum of the paramagnetic anion is complicated by the simultaneous presence of a high number of "normal" hyperfine lines along the principal axes and also by several off-axis extra features which have complicated previous interpretations of the A(yy) and A(zz) components. By adopting a suitable adsorption procedure which suppresses the superoxide speciation, using a highly crystalline MgO material and controlling the isotopomer composition through appropriate O-17 enrichments, the resolution of the EPR spectrum has been dramatically improved. Analysis of the H-1 superhyperfine structure (\A(H)\/beta(e)g=[3.9,2.2,1.3]G), resulting from a dipolar interaction between the adsorbed O-2(-) anion and a neighboring OH group, and positions of the extra absorption lines in the spectrum, have provided us with auxiliary sources of information to determine for the first time the complete O-17 hyperfine tensor (A(O)/beta(e)g=[-76.36,7.18,8.24] G). The tensor has been analyzed in detail using a localized spin model. The spin density is shared among the 2p(pi)(x)(0.495), 2p(x)(y)(-0.024) and 2s(0.011) orbitals. The total spin density on O-2(-) indicates that a complete surface electron transfer from the F-S(+)(H) center to dioxygen occurs upon adsorption, in line with recent ab initio calculations.