3D printing of metals, such as laser powder bed fusion (LPBF) printing of stainless steels, often leads to elevated oxygen content in the alloy substrate relative to conventional processing routes. Here we show that the extremely rapid cooling rate (106–107 K/s) during LPBF processing of austenitic stainless steel can trap a considerable fraction of oxygen and other elements in the interstitial sites of the metal lattice, at concentrations far exceeding the room temperature solubilities. High resolution characterization and atomistic simulations with density functional theory reveal that oxygen and other elements exist in octahedral interstitial sites of the metal lattice and bond with their neighboring metal atoms. Our findings suggest that additive manufacturing can be a potential strategy of incorporating beneficial interstitial elements into a metal substrate. Given the well-known effect of interstitial elements in conventional alloys, significant improvement of the physicochemical properties of printed alloys is possible.