The reliability of AlGaN/GaN HEMTs is strongly influenced by oxygen introduced during processing (intrinsic) and operation (extrinsic). Thick SiN passivation reduces ingress of extrinsic oxygen and moisture, but increases parasitics and reduces RF performance. This work systematically separates the roles of intrinsic and extrinsic oxygen by combining different plasma pre-cleans (plasma 1, plasma 2) with either a hot-plate (HP) or a quartz-tube-furnace (QTF) anneal, using 100 nm SiN passivation and benchmarking against 325 nm and 825 nm SiN devices. On-state reliability tests show that 100 nm SiN devices with plasma 1/QTF exhibit reliability comparable to 825 nm devices and clearly superior to 325 nm devices. Stress in air and N2 confirms that intrinsic oxygen dominates early degradation, while extrinsic oxygen accelerates it at later times. STEM-EDX reveals strong oxygen accumulation and pit formation at the gate foot only for nonoptimised flows. This anneal has been identified as the key step for improving reliability, enabling thin passivation without compromising reliability.