Developing effective photodynamic theranostics for real-time monitoring of oxygen saturation at a specific target for photodynamic therapy (PDT) is highly desirable, but remains a great challenge owing to the separation of monitoring units and PDT units in existing methods. In this study, we report the real-time monitoring of tissue oxygen saturation (StO(2)) during PDT using an 808 nm-excited upconversion nanoagent, UCNP-RB-RGD, fabricated by loading the photosensitiser rose bengal (RB) into amphiphilic upconversion nanoparticles, followed by conjugation with the tumour-targeting peptide c(RGDyK). Using the as-prepared UCNP-RB-RGD, we demonstrated that the upconversion luminescence at 645 nm and 540 nm under 808 nm laser irradiation could be used to simultaneously monitor StO(2) and activate RB for PDT. Furthermore, benefitting from this new photodynamic theranostic approach, the PDT efficacy was significantly enhanced by controllable adjuvant oxygen inhalation to resist the consumption of oxygen. The StO(2) monitoring unit and PDT unit were integrated into the single nanoagent UCNP-RB-RGD, providing high spatiotemporal consistency for the photo-controlled monitoring of StO(2) and localised activation of PDT; this system has potential for effective and accurate personalised PDT.