An amphiphilic graft copolymer (P3) composed of poly(oligo (ethylene glycol) methacrylate) (POEGMA) as a hydrophilic segment and a polystyrene (PS) as a hydrophobic chain was prepared. P3 was used to incorporate a highly efficient hydrophobic oxygen probe platinum(II)-5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (PtTFPP) in its micelles to enable the application of PtTFPP in aqueous solution for biosensing. For achieving ratiometric oxygen sensing, a hydrophobic conjugated polymer (CP) was used as a fluorescence resonance energy transfer (FRET) donor for PtTFPP-the FRET acceptor. Results showed that diameters of these nano-oxygen sensors ranging from 50 to 60 nm and FRET efficiency as high as 98% could be achieved. Further, FRET contributed to the brightness of PtTFPP in micelles. High quantum yields of PtTFPP in non-FRET micelles and FRET micelles under nitrogen were achieved to be 0.109 and 0.231, respectively. The sensors were found to be able to measure oxygen concentrations from 0.082 to 40.9 mg/L at room temperature with linear Stern-Volmer constants. These sensors were also demonstrated to be suitable for monitoring extracellular oxygen consumption of E. Coll and HeLa cells. This study illustrated a feasible approach to achieve highly efficient ratiometric oxygen sensors without specific structure modification of the efficient PtTFPP probe, however using an amphiphilic graft copolymer approach.