Polymers are susceptible to oxidation, and its impact on their mechanical behavior presents a significant challenge for their long-term performance. Interestingly, in simple tension UV-aged polyethylene plates do not show any significant changes in modulus. But their nano-indentation data reveal up to a twofold increase in near-surface stiffness with increasing exposure time. To understand this apparent conflict, we combined mechanical testing and electron microscopy of pristine and UV-aged unstabilized semicrystalline polymer films and plates. Our study provides new insights into the complex relationship between two competing mechanisms that concurrently appear during the UV aging of semicrystalline polymers: oxidation-induced stiffening and weakening. The former mechanism, oxidation-induced stiffening, is likely caused by UV-induced chemi-crystallization and cross-linking, which increases local crystallinity. On the other hand, oxidation-induced weakening is due to the profuse cracking that accumulates with exposure time. Our findings have important implications for the development of predictive models of polymer aging and improving their long-term performance.