New findings are presented relating to the optimal choice of gate insulators in organic field effect transistors (OFET). It was recently found that some organic semiconductors operate better when low-k materials are used in the gate. This is quite contrary to the conventional trend to use high permittivity dielectrics for low voltage operation. Interaction between the insulator and the semiconductor materials plays an important role in carrier transport. On one hand, the insulator is often responsible for the morphology of the semiconductor layer, but on the other hand it can also change the distribution of states by local polarisation effects. Carrier localisation is enhanced by insulators with large permittivities, due to the random dipole field present at the interface. We have investigated this effect on a number of disordered organic semiconductor materials and show here that the use of low-k materials may lead to improvements in mobility, reduced temperature activation and hysteresis. In particular, the behaviour of the threshold voltage is interesting. The differences in the underlying physics compared to the case of FETs based on band-like semiconductors, is also discussed.