<p>Alumina-forming austenitic (AFA) steels exhibit exceptionally high-temperature performance; however, their high chromium content (&gt;12 wt.%) promotes the precipitation of a brittle <i>σ</i>-phase, which degrades their mechanical and oxidation properties. This study investigates the microstructure and oxidation resistance of novel cast low-chromium (5 wt.%) AFA steels with varying aluminum contents (0, 4, 6, and 8 wt.%). The alloys were homogenized at 1200&#xa0;°C for 4 hours and subjected to isothermal oxidation in dry air at temperatures ranging from 600 to 800&#xa0;°C for 72 hours. Microstructural and compositional analyses were performed using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray diffraction. The results indicate that a fully austenitic microstructure was maintained with up to 4 wt.% Al, while 8 wt.% Al resulted in a dual-phase austenitic + B2–NiAl structure. At 600&#xa0;°C, a minimum of 6 wt.% Al was required to form a continuous, protective Al₂O₃ layer without the precipitation of needle-like B2–NiAl phases. In contrast, at 800&#xa0;°C, alloys with less than 8 wt.% Al exhibited poor oxidation resistance and massive internal oxidation, attributed to the higher oxygen diffusivity. The 8 wt.% Al alloy formed a protective Al₂O₃ scale, demonstrating the highest oxidation resistance (<i>K</i>_<i>p</i> = 1.52 × 10⁻¹¹ g²·cm⁻⁴·s⁻¹). This scale effectively prevented further oxidative degradation and eliminated internal oxidation. Furthermore, microstructural analysis confirmed the complete absence of the detrimental <i>σ</i>-phase following homogenization and oxidation. This work demonstrates that a significant reduction in chromium content is feasible without compromising oxidation performance, paving the way for a new class of oxidation-resistant alloys for advanced high-temperature applications.</p>