Understanding the adsorption and organization of nanocelluloses at oil/water interfaces is crucial to develop a promising route to fabricate functional materials from the bottom-up. Here, we prepare acetylated cellulose nanofibrils (CNFs) with 2 degrees of substitution and investigate their assembly behavior at the oil/water interface. We study the adsorption process by tracking the dynamic interfacial tension using pendant drop tensiometry and further characterize the viscoelasticity of the CNF interfacial films as a function of ionic strength. The results show that the adsorption of the CNFs at the interface is dominated by energy barriers associated with electrostatic repulsion. With the addition of NaCl, the fibrils are rapidly accumulated at the oil/water interface and jammed into a solidlike film. The overall accumulation of the fibrils is related to the competition between van der Waals attractive forces and electrostatic repulsive forces according to the Detjaguin-Landau-Verwey-Overbeek theory. By screening on the fibril-fibril and fibril-interface electrostatic repulsive forces, the salt addition facilitates the formation of packed fibril clusters and the development of the clusters into a solidlike film. Moreover, the salt addition is assumed to trigger an abrupt density fluctuation in the vicinity of the interface (the formation of locally dense clusters and voids), leading to an increase in brittleness of the film.