The active control of flow-induced oscillations, specifically, vortex-induced oscillations of circular cylinders and galloping oscillations of circular cylinders and galloping oscillations of a square prism, are considered. In the case of vortex-induced oscillations, the vibrating cylinder is modeled as a single-degree-of-freedom (sdof) linear damped oscillator and the fluid motion as a Rayleigh oscillator. Galloping oscillations are modeled as a sdof limit-cycie system with cubic and higher order non-linearities in the velocity term. In formulating the control method it is assumed that the fluid interaction is an external excitation/disturbance on the structure. A suitable disturbance model is chosen based on the spectral content of the disturbance. For the problems considered here, the disturbance model assumes the form of a harmonic oscillator the frequency of which, in the case of vortex-induced oscillations, is equal to the natural frequency of the structure or the "lockin" frequency, and in the case of galloping oscillations, is equal to the limit-cycle frequency. A disturbance counteracting control law is synthesized by using a Luenberger observer incorporating the structure and disturbance dynamics. It is shown that using this approach one is able to completely suppress the fluid-induced oscillations of the structure.