The weakest phase of concrete is the interfacial transition zone (ITZ), which has a higher porosity and is more susceptible to microcracks. This study investigates the ITZ in fiber-reinforced concrete by utilizing oil shale ash (OSA) as a supplementary cementitious material (SCM) and macro-basalt fibers (BFs) to improve the properties of the concrete and reduce its carbon footprint. Six substantial blends in with 10%-30% OSA and a consistent 8.0 kg/m³ BF portion were tried. SEM pictures and mechanical tests showed that OSA further developed holding and densified the ITZ microstructure, keeping up with flexural strength up to 20% substitution and diminishing compressive strength by 6.7%, while bringing down the an Earth-wide temperature boost potential by 19.24 kg CO2 identical with 10% OSA. Because of their inert filler behavior and incomplete reaction, higher OSA replacements had a negative impact on mechanical properties.