Liquid metal embrittlement (LME) refers to the phenomenon wherein the ductility of a solid metal markedly decreases upon contact with a liquid metal. This issue is a critical constraint in the development of lead-cooled fast reactors (LFRs). The commonly accepted mechanism involves the adsorption of liquid Pb–Bi atoms at the crack tips of solid metals, which alters the bonding interactions, lowers the critical stress for rupture, and ultimately causes embrittlement. 9Cr ferritic/martensitic steels are considered potential candidate materials for LFRs. In this study, the slow strain rate tensile behavior of P92 steel was investigated in liquid lead–bismuth eutectic (LBE), focusing on the effects of temperature (150–500 °C), strain rate (4 × 10⁻⁷–4 × 10⁻⁴ s⁻¹), and dissolved oxygen concentration (from 10⁻¹⁰ mass fraction to oxygen saturation). The results showed that no significant embrittlement occurred in oxygen-saturated conditions. In contrast, high embrittlement sensitivity was observed in oxygen-deficient LBE at low strain rates. At high oxygen concentrations, a protective oxide film forms on the surface, effectively isolating the solid metal from liquid LBE, thereby preventing embrittlement. However, this protective effect diminishes significantly if the integrity of the oxide film is compromised. High strain rates promote mechanical damage to the oxide layer, which facilitates LME. In oxygen-poor LBE, the protective oxide film barely forms, leading to direct exposure of the steel matrix to the liquid LBE. This exposure causes dissolution corrosion, where pits formed by metal dissolution act as crack initiation sites owing to localized stress concentration, promoting LME. Interestingly, excessively high strain rates can accelerate crack propagation to such an extent that the crack tip advances before sufficient adsorption of Pb/Bi atoms occurs. This rapid progression inhibits the embrittling action of the liquid metal, thereby reducing the LME effect under low dissolved oxygen conditions.