The　thermal　instability　of　organic-inorganic　halide　perovskite　solar　cells　(PSCs)　is　one　of　the　most　important　factors　restraining　their　commercialization.　It　has　recently　been　reported　that　modifications　at　the　interface　between　the　electron　transport　layer　(ETL)　and　the　perovskite　layer　could　effectively　promote　the　long-term　thermal　stability　of　high-efficiency　PSCs.　However,　few　studies　have　disclosed　the　effect　of　the　microstructure　of　ETLs,　such　as　SnO2　or　TiO2,　on　the　thermal　degradation　pathway　of　the　perovskite　layer　at　nano-,　or　even　atomic-scale,　resolution.　In　this　work,　a　variety　of　characterization　techniques　were　used　to　detect　the　diffusion　of　the　oxygen　element　from　SnO2/TiO2　layers　into　organic-inorganic　hybrid　perovskite　(OIHP)　layers,　which　was　accompanied　by　the　thermal　decomposition　of　MAPbI(3)　and　FA(0.9)Cs(0.1)PbI(3)　films.　Moreover,　a　preferential　thermal　decomposition　of　the　perovskite　layer　at　the　SnO2-perovskite　interface　was　confirmed　to　be　an　adverse　factor　inducing　the　performance　degradation　of　PSCs.　First-principles　calculations　further　elucidate　that　dangling　bonds　at　the　SnO2　or　TiO2　ETL-perovskite　interface　down-regulate　the　oxygen　vacancy　formation　energy.　Furthermore,　a　low　segregation　energy　for　oxygen　diffusion　(0.19-0.55　eV　and　0.26　eV　at　SnO2　and　TiO2-perovskite　interfaces,　respectively)　accelerates　the　transfer　of　the　oxygen　element　from　SnO2　or　TiO2　into　OIHP　layers　and　promotes　the　transfer　of　protons　in　the　perovskite　layer.　Furthermore,　the　fabrication　of　planar　OIHP　solar　cells　based　on　an　O-2-plasma　treated　ETL　SnO2　layer　effectively　improved　their　photoelectric　performance　and　thermal　stability,　further　confirming　the　important　role　of　interfacial　oxygen　in　modulating　the　stability　of　PSC　devices.