Full-scale measurements of the motions of the Girassol FPSO are available over a one-year period. During this same period also vessel draft, wave height and wave direction have been recorded. This allows the calculation of motion transfer functions as function of the FPSO loading condition. A comparison has been made between the measured motion transfer functions and the calculated motion transfer functions based on diffraction theory, taking into account the measured wave spreading. Viscous roll damping has been added in the calculation to obtain maximum agreement between measured and calculated RAO’s. The results show that wave spreading is a very important factor on how the vessel behaves and should be taken into account when evaluating measured full-scale motion RAO’s. This paper addresses the full-scale monitoring campaign in some detail and the related issues. Furthermore it describes the technique how measured wave spreading is taken into account in the calculation of theoretical motion RAO’s. It is shown that the agreement between calculated and measured RAO’s is greatly improved by the use of measured wave spreading in the calculation. The levels of viscous roll damping found in this tuning process can be described as function of loading condition and sea state. The viscous roll damping found in this process also shows good agreement to model test results performed earlier on this same FPSO.