Resonant soft x-ray reflectivity (R-SoXR) is employed to determine the interfacial widths of the semiconductor/dielectric interface in P(NDI2OD-T2)-based top-gate organic field-effect transistors (OFETs). It is shown that the deposition of a polymer dielectric on top of a semiconducting polymer layer can affect the interface structure, even when cast from an orthogonal solvent. The observed differences in the interfacial widths for different dielectrics explain the insensitivity of OFET performance to dielectric choice for OFETs fabricated using an identical fabrication protocol. The R-SoXR results demonstrate that differences in the physical interface structure should be taken into account when considering the influence of polymer dielectrics on the performance of all solution-processed OFETs. Specifically, the importance of the choice of solvent for the deposition is highlighted.