Purpose: To map the cerebral metabolic rate of oxygen (CMRO2) by estimating the oxygen extraction fraction (OEF) from gradient echo imaging (GRE) using phase and magnitude of the GRE data. Theory and Methods: 3D multi-echo gradient echo imaging and perfusion imaging with arterial spin labeling were performed in 11 healthy subjects. CMRO2 and OEF maps were reconstructed by joint quantitative susceptibility mapping (QSM) to process GRE phases and quantitative blood oxygen level-dependent (qBOLD) modeling to process GRE magnitudes. Comparisons with QSM and qBOLD alone were performed using ROI analysis, paired t-tests, and Bland-Altman plot. Results: The average CMRO2 value in cortical gray matter across subjects were 140. +/- 14.9, 134.1 +/- 12.5, and 184.6 +/- 17.9 mu mol/100 g/min, with corresponding OEFs of 30.9 +/- 3.4%, 30.0 +/- 1.8%, and 40.9 +/- 2.4% for methods based on QSM, qBOLD, and QSM + qBOLD, respectively. QSM + qBOLD provided the highest CMRO2 contrast between gray and white matter, more uniform OEF than QSM, and less noisy OEF than qBOLD. Conclusion: Quantitative CMRO2 mapping that fits the entire complex GRE data is feasible by combining QSM analysis of phase and qBOLD analysis of magnitude.