Stability of π-conjugated organic materials remains a critical issue for applications in which these materials and devices based on them are exposed to ambient conditions. Particularly, the initial steps of the reversible and irreversible degradation by molecular oxygen exposure are still not fully explored. Here we present a theoretical study using density functional theory (DFT) to investigate the oxygen effects on the electronic properties of poly(3-hexylthiophene-2,5-diyl) (P3HT). Our results show that trap-states are introduced in the energy gap between the highest occupied and the lowest unoccupied molecular orbitals by the O₂ molecule and both singlet and triplet states can be formed irrespectively of the existence of chain defects. A crossing between the potential energy surfaces of singlet and triplet states was observed for smaller distances of the oxygen molecule to the nearest thiophene ring, which was identified as being the first step towards irreversible degradation.