Abstract: Maraging steels are ultrahigh-strength, low-carbon steels requiring strict control of impurity elements to ensure optimal strength and toughness. This study aims to elucidate the role of oxygen content in controlling oxide inclusions and cryogenic toughness in laser powder bed fusion (L-PBF) fabricated maraging steels. Two types of powders were used: vacuum induction gas atomization (VIGA) powder with 0.034wt% oxygen and plasma rotating electrode process (PREP) powder with 0.016wt% oxygen. The PREP deposit exhibited finer and more dispersed Al₂O₃ inclusions (average size: (37 ± 11) nm; number density: 7.6 × 10¹⁸ m⁻³) compared to the VIGA deposit ((59 ± 28) nm; 6.5 × 10¹⁸ m⁻³). As a result, the PREP specimens demonstrated significantly improved impact toughness—138 J at 23°C and 65 J at −196°C—representing 53.3% and 47.8% increase over the VIGA specimens, respectively. This difference is due to the lower oxygen content in PREP, leading to lower-temperature nucleation with a reduced nucleation barrier. In addition to quantitatively evaluating the oxygen inclusion–toughness relationship, a thermodynamic model was developed to capture the nucleation and evolution of nanoscale oxides under the rapid thermal cycles characteristic of the L-PBF molten pool, which enables prediction of the size and number density evolution of oxide inclusions under different oxygen levels. These findings offer insights for oxygen-level control and powder design strategies in additive manufacturing of maraging steels.