When Oxygen Matters: Surface Chemistry and Early-Stage Sintering: The sintering behaviour of powder-based metallic systems is strongly influenced by surface chemistry, with oxygen playing a particularly critical role. This contribution examines how surface oxides affect diffusion mechanisms, neck formation, and densification during the early stages of sintering. Key chemical reactions occurring at particle surfaces are discussed, highlighting how subtle differences in oxygen content and oxide stability can lead to markedly different sintering responses across metallic systems. Understanding and controlling these effects is essential for optimizing sintering routes and achieving reliable and reproducible densification.