The rapid development and increasing demand for concrete in the construction industry have led to the uncontrolled exploitation of natural aggregates and the emission of greenhouse gases. Additionally, the disposal of large quantities of industrial waste, such as oil palm shells from palm oil mills and fly ash from coal power plants, has raised significant ecological concerns. Utilizing these solid wastes as alternative materials in the production of lightweight concrete offers practical and sustainable advantages. This research investigates the effects of varying dosages of fly ash (0%, 5%, 10%, 12.5%, 15%) as a supplementary cementitious material in concrete production, using oil palm shell as a fine aggregate substitute. The fresh and hardened properties of the concrete mixtures were evaluated and analyzed. The experimental results indicate that while the addition of fly ash reduced the early-age strength of the concrete, a 10% replacement of cement with fly ash by weight achieved the highest compressive strength at 28 days. Increasing the fly ash content also improved other mechanical properties, such as water absorption and ultrasonic pulse velocity. However, the workability of the concrete was adversely affected. Overall, the findings demonstrate the potential for incorporating industrial wastes, such as oil palm shell and fly ash, to produce environmentally friendly lightweight concrete.