Three-dimensional oscillations of a supersonic opposing jet flow around a hemispherical nose are investigated through numerical analysis that consists of an efficient implicit algorithm. The results show that axially asymmetric oscillations are reproduced in both stable and unstable region, and that frequency of oscillation in unstable region agrees well with existing experimental results. On the basis of the numerical results, the mechanism of oscillation in unstable region suggested in former study of the author is quantitatively validated, and the mechanism of oscillation in stable region is newly presented here. Also in unstable region, unsteady aerodynamic force on the hemispherical nose is shown to be axially asymmetric and rotating in circumferential direction.