[en] Using magnetization, conductivity and x-ray scattering measurements, we demonstrate that the giant magnetoresistance of the oxygen-deficient ferrite SrFeO_2.875±0.02 is a consequence of the coupling between the charge and spin order parameters and the tetragonal to monoclinic structural distortion. Upon cooling the sample at T ≃ 120 K we find a shoulder in both field-cool and zero field cool magnetization data and the simultaneous appearance of incommensurate structural satellites observed using x-ray diffraction. These satellites are shown to be due to incommensurate charge ordering with the high temperature delocalized Fe${}^{3.5+}$ ions becoming localized with a charge disproportion forming an incommensurate charge-ordered phase. Strong resonant enhancement of these satellites at the Fe L _III absorption edge confirms that this charge ordering is occurring at the Fe(2) sites. Further cooling increases the charge order correlation until T ≃ 62 K where there is a full structural transition from the tetragonal phase to a mononclinic phase. This causes a jump in the charge order wavevector from an incommensurate value of 0.610 to a commensurate ground state position of 5/8. This first-order structural transition displays considerable hysteresis as well as dramatic reductions in the magnetization, resistivity and magnetoresistance. The transition also causes an antiferromagnetic spin-ordering with a doubled unit cell along the c-axis. Well as observing new commensurate magnetic reflections at the Fe_III edge we also observed resonant enhancement at the oxygen K-edge showing considerable hybridization between the Fe 3d and oxygen 2p states at low temperatures. Our results show that the formation of a magnetic long-rage ordered ground state drives the charge ordering from an incommensurate ordering to a commensurate ground state. This is evidence of a strong coupling between the magnetic and charge order parameters which is the basis for the unusual magnetoresistive effects observed at the transition. (paper)