Adsorption of proteins to fluid interfaces is critical in many industries, scientific disciplines, and biological processes. However, the structural transitions of proteins upon adsorption and the effect of the hydrophobic subphase, such as oil, have received little attention. Herein, we present a comprehensive study on the effect of the hydrophobic subphase on the adsorption behavior of globular and random coil proteins. The adsorption of proteins is limited by their structural stability, and accordingly, is faster for less stable globular proteins and fastest for random-coil proteins. Protein adsorption is slower at more polar oils, regardless of the protein type, structure, and stability. Moreover, we found a correlation of oil polarity and the induced surface pressure of proteins, which seems universally applicable and describes the experimental data of over 30 previous studies. The model works for all commonly applied subphases, with the exception of oils that chemically react with proteins (e.g. octanal) and air, due to the lack of hydrophobic interactions. These results foster our understanding of protein adsorption and allow the prediction of protein unfolding depending on protein-subphase interactions. (C) 2020 The Author(s). Published by Elsevier Inc.