This paper investigates open-loop control of the mixing between an inner stream and an outer stream in a coaxial combustor geometry. The inner and outer air streams enter the combustor geometry at different temperatures and mimic a gaseous fuel and combustion air stream respectively. Of specific interest in this study is the behavior of the coherent structures in the coaxial jet streams, and the manipulation of these coherent structures with controlled perturbation to enhance jet-spreading and scalar-mixing. The spectra of the unforced flow reveal the presence of a dominant coherent mode at 100 Hz (identified as the fundamental mode fo), as well as 150 Hz (3 fo/2) and 300 Hz (3 fo). Single-mode forcing of both the inner-jet and the outer-jet at 100 Hz, 150 Hz, 300 Hz, and 1 kHz is explored, and attention is focused on the spreading of the inner-jet shear layer and the outer-jet shear layer. It is observed that the 300 Hz mode shows the greatest enhancement in the spreading rate of the velocity shear-layer in the near field (x/D < 1), while downstream of x/D = 1, the 100 Hz appears to show the most significant effect. Scalar mixing is also significantly enhanced, with the 300 Hz forcing showing the largest enhancement. Dual-mode forcing is also investigated with 100 Hz inner-jet, 300 Hz outer-jet forcing and 300 Hz inner-jet, 100 Hz outer-jet forcing. The 100 Hz inner-jet, 300 Hz outer-jet forcing is shown to lead to greater enhancements in scalar mixing man all other cases considered.