This research aims to investigate the durability and behavior of Jordanian oil shale ash (OSA) as a substitute material for ordinary Portland cement (OPC) and Portland Pozzolana cement (PPC) in mortar under sulfate attack. Mortar samples with replacement ratios of 0%, 10%, 20%, and 30% of OSA were utilized in this investigation. The study is divided into two stages. In the first stage, the effect of OSA replacement on the mechanical properties and hardening time of mortar samples is examined over a curing period of 3, 7, 14, and 28 days using potable water. The second stage focuses on studying the durability of mortar samples with the aforementioned OSA replacement ratios. These mortar samples are fully immersed in a sodium sulfate solution with a concentration of 5% and subjected to wet and dry cycles for degradation at 1, 6, 12, 18, 24, 30, and 36 days. Various analyses were conducted, including compressive strength testing, SEM analysis, mass loss measurement, ultrasonic pulse velocity assessment, relative dynamic modulus of elasticity (RDME) evaluation, and visual inspection. For OPC-based mortars, compressive strength ranged from 44.00 to 49.75 MPa (0% OSA), 42.88 to 48.29 MPa (10% OSA), 44.50 to 50.88 MPa (20% OSA), and 31.00 to 36.53 MPa (30% OSA). For PPC-based mortars, compressive strength ranged from 36.00 to 42.32 MPa (0% OSA), 39.73 to 43.60 MPa (10% OSA), 44.97 to 46.06 MPa (20% OSA), and 22.35 to 28.18 MPa (30% OSA). Specimens with 0%, 10%, and 20% OSA showed continued increases in compressive strength, UPV, and RDME throughout exposure, while 30% OSA specimens showed deterioration after 24 days (OPC) and 18 days (PPC), with mass losses increasing from 0.52% to 8.4% (OPC) and 0.75% to 7.8% (PPC). The results indicate that while moderate OSA replacement (up to 20%) may be viable under sulfate exposure, 30% replacement leads to significant deterioration. However, the absence of non-sulfate controls prevents definitive separation of continued hydration from sulfate resistance. These findings provide practical guidance for the use of OSA in sulfate-prone environments.