In this paper, oscillation modes of single jet, twin jet, and multijet with square configuration were investigated by using optical and acoustical observations. In the case of a single jet, axisymmetrical, helical, and lateral oscillation modes appeared with increasing pressure ratio of the jet. In the case of twin jets, two counter rotating helical oscillation modes and reversely lateral oscillation modes along a line through the center line of the two jets were observed for two different pressure ratios. In the case of multijets with square configuration, the quadrupolelike oscillation mode, four counter rotating helical oscillation modes, and a symmetrical oscillation mode were observed with increasing pressure ratio. The first and the last oscillation modes of the multijet with square configuration are unexpected based on observations for a single jet.