The field of organic field-effect transistors (OFETs) has matured over the past two decades and devices with charge carrier mobilities of several cm2/Vs have been demonstrated. Nevertheless, to enter commercial products, the operational stability of OFET technology also needs to be addressed. Herein, two potential degradation mechanisms occurring in N,N′-di-1-naphthalenyl-N,N′-diphenyl [1,1':4′,1″:4″,1‴-quaterphenyl]-4,4‴-diamine (4P-NPD)-based OFETs are investigated. Electrical transfer characteristics show that photo-induced oxidation of the organic thin-film decreases the hole mobility and increases the threshold voltage. The inverse hole mobility depends approximately linearly on the concentration of photo-induced defect states, which is in agreement with the Multiple Trapping and Release model. This implies that the created defects act as hole traps located energetically above the highest occupied molecular orbital level. In contrast, electrical stress increases the threshold voltage without affecting the hole mobility. This behavior is related to charge trapping, which probably take place in the gate dielectric as indicated by a relatively long trapping time of ∼102 s. Time-resolved photoluminescence spectroscopy, which proved to be sensitive to both mobile and trapped charges, shows that the threshold voltage shift is caused exclusively by charge accumulation. The obtained results broaden the fundamental knowledge about the degradation processes occurring in OFETs, which is highly relevant for increasing their stability.