The design of Offshore Wind Turbines (OWTs) relies on integrated simulation tools capable of predicting the system dynamic characteristics and the coupled loads and responses. Despite all efforts to develop accurate integrated models, these often fail to reproduce the measured natural frequencies, partly due to the modelling of the foundation. Several foundation models and calibration approaches have been proposed and compared with small or large scale field tests, where only the soil and the foundation are included. However, there is a lack of more integral validation where the interaction between the foundation and the structure is taken into account. The paper investigates the impact of the foundation model and calibration approach on the simulated response of a monopile-based OWT installed in the North Sea by comparing simulations and full-scale field data. The OWT structure and the environmental actions are implemented in the aero-servo-hydro-elastic code 3DFloat. Two foundation models and two calibration approaches are evaluated. The results indicate that, with a conceptually correct foundation model and a realistic calibration, it is possible to match the measured natural frequency and predict accurate fatigue loads. More accurate predicted loads will reduce uncertainties in the estimated fatigue lifetime and therefore reduce risk in the design.