To investigate the dynamic response of a floating moored oscillating water column device under realistic sea states, a focused wave technique was developed in this study to generate user-defined wave trains in a wave basin equipped with a piston-type wavemaker. The experimental setup allowed for simultaneous measurements of the designed focused wave trains, internal chamber wave elevations and air pressure, dynamic tendon response and the model's global motion responses. Based on the experimental results it was found that to accurately replicate the theoretical wave time series, a minimal focal reconstruction of 200 components was required which resulted in a focused wave regeneration accuracy greater than 93% with strong repeatability. The tested model showed typical motion responses to that of generalised tension leg platform systems with significant surge offsets along with stiff heave and pitch motions. Applying vertical tendons to the system allowed for the floating offshore device to be exposed to deeper waters, and hence the larger energy source associated with deep water ocean waves.