The interfacial transition zone (ITZ) is the weakest phase in concrete, characterised by higher porosity and being prone to microcrack formation. Additionally, the ITZ is created when dispersed fibre reinforcement is present. Although fibres improve flexural strength, they can negatively impact other properties. This research investigates the ITZ of fibre-reinforced concrete where macro-basalt fibres (BFs) and oil shale ash (OSA), as an SCM, were used with the aim of modifying the properties of concrete, enhancing the ITZ, and reducing its carbon footprint. Six different concrete mixes with OSA doses between 10% and 30% and a constant BF dose of 8.0 kg per 1 m3 of concrete were prepared and tested. The ITZ was analysed with SEM images and verified through its mechanical properties. The results showed that the presence of OSA improved bonding and densified the microstructure of the paste, especially in the ITZ, resulting in a nearly constant flexural strength at up to a 20% replacement and only a 6.7% decrease in compressive strength while reducing the global warming potential by 19.24 kg CO2 equivalent in the mix with 10% OSA replacement. Higher replacement ratios had a negative impact on the mechanical properties, as the OSA had not reacted entirely and served partly as an inert filler.