Advancements in organic electrochemical transistor (OECT) applications have been largely driven by the development of organic electronic materials that allow for simultaneous ionic and electronic transport in the bulk of their films. These studies focus on achieving high steady-state OECT performance, governed by the electronic charge mobility and the capacitance of the polymer film in the channel, and an often underlooked property is the long-term operational stability. In this work, we present a strategy to improve the performance of p-type OECTs along with operational stability via two additives, i.e., a high-boiling-point solvent (chlorobenzene) and a Lewis acid (tris(pentafluoro phenyl)borane). Addition of a small amount of a cosolvent additive changes the arrangement of glycolated thiophene-based copolymer chains on the substrate toward a direction that allows for more efficient hole transport. The Lewis acid, on the other hand, boosts the OECT stability, mainly by preventing oxidative degradation. Using both additives in the solution grants OECTs with high operational stability and performance through changes in the film microstructure and the polymer’s sensitivity to oxygen. This study highlights the use of additives as a means to enhance the OECT figure of merits without the need for new polymer synthesis.