The defining innovation behind Ketos, which would later become standardized as O1, was its ability to harden effectively in oil rather than water. When the American Iron and Steel Institute (AISI) eventually formalized tool steel classifications, the designation "O1" simply identified it as the first standardized oil-hardening tool steel. While that naming was straightforward, the technology behind it was a significant advancement. Because oil cools steel more gradually than water during quenching, it greatly reduces the internal stresses that often cause distortion or cracking. Toolmakers could produce dies, punches, gauges, and cutting tools that, after heat treatment, were much closer to their intended dimensions, reducing finishing work, improving consistency, and lowering production costs.

As American manufacturing expanded, O1 found its way into countless industrial applications, including blanking dies, punches, reamers, taps, broaches, gauges, woodworking cutters, shear blades, precision measuring tools, and machine-shop fixtures. Its blend of wear resistance, machinability, dimensional stability, and straightforward heat treatment made it a favorite among tool-and-die makers, where reliability often outweighed maximizing any single performance characteristic.

Its transition into the knife world was a natural progression. The same qualities that made O1 excel in industrial tooling translated exceptionally well to cutting instruments. Knifemakers discovered that it could be ground exceptionally thin without sacrificing durability, sharpened quickly to razor-like keenness, and easily maintained with traditional sharpening stones. Rather than relying on large, hard alloy carbides to resist wear, O1 develops a fine microstructure dominated by small cementite carbides, allowing skilled sharpeners to achieve exceptionally refined cutting edges that many high-alloy steels struggle to match.

Among the earliest and most influential companies to embrace O1 was Randall Made Knives. Beginning in the late 1930s, Bo Randall selected the steel for many of his handmade fixed blades, helping establish its reputation among hunters, outdoorsmen, soldiers, and collectors alike. Randall knives earned legendary status during World War II, the Korean War, and Vietnam, introducing generations of servicemen to O1's dependable cutting performance under demanding conditions. More than eight decades later, Randall continues to offer numerous models in O1, making it one of the longest continuously used knife steels still in production.

Beyond Randall, O1 steadily became a popular steel for the custom knifemaking community. Bladesmiths valued how well it forged, how predictably it responded to thermal cycling, and how forgiving it remained throughout the heat-treating process. Stock-removal makers appreciated its machinability, while hobbyists discovered that quality results could often be achieved with relatively modest equipment. For decades, O1 has remained one of the most frequently recommended steels for beginning bladesmiths, providing an approachable introduction to both forging and heat treating without sacrificing real-world performance.

As metallurgy advanced throughout the second half of the twentieth century, O1 faced increasingly capable competition. Air-hardening steels such as A2 offered greater toughness with reduced distortion, while D2 delivered dramatically improved wear resistance through its higher chromium carbide content. Stainless knife steels including ATS-34, 154CM, VG-10, CPM S35VN, MagnaCut, and many others eventually combined edge retention with corrosion resistance that would have seemed impossible when Ketos first appeared in 1905.

More than 120 years after its introduction as Halcomb Steel's Ketos alloy, O1 remains one of the oldest blade steels still in regular production. Over the course of a century, it has served machinists, tool-and-die makers, soldiers, hunters, outdoorsmen, and countless custom knifemakers, adapting to changing industries without ever losing the qualities that made it successful in the first place. Although newer steels have expanded the boundaries of edge, O1 continues to have a respected place in modern knifemaking, proving that a thoughtfully engineered alloy can remain relevant long after the technologies that followed it.