Characterising soft biological tissues outside the linear viscoelastic regime is challenging due to their complex behaviour. In addition, the viscoelastic properties of tissues have been shown to be sensitive to sample preparation and loading regime resulting in inconsistent data varying by orders magnitude in the literature. This paper presents a novel technique to characterise the non-linear behaviour of tissues which uses Fourier Transformation to decompose the stress output waveform under large amplitude oscillatory shear (LAOS) into harmonic contributions. The effect of varying preload, the compressive strain exerted on a liver tissue specimen prior to shear testing to minimise slip, was also investigated. Results showed that in the linear regime, preload affects the viscoelastic response of liver. Histological analysis indicated that there were structural changes as a result of the preload that may be linked to the differences in observed behaviour. Fourier analysis was used to extract the first and third harmonic components of the shear moduli at large strain. At 50% shear strain, a change in the third harmonic component of the shear moduli was accompanied by a marked change in the micro-structural arrangement of the sinusoids. This paper demonstrates a method of efficiently characterising soft biological tissues under large amplitude oscillatory shear under combined loading.