It was found that the dielectric properties of BaTiO3 films on Ni foil substrates varied with oxygen partial pressure and composition. 200 nm thick Mn doped BaTiO3 films annealed at 1000°C in 10 -12 atm po2 showed a dielectric constant of 950 and low loss up to 250kV/cm bias electric field. Mn doping may trap electrons introduced by oxygen vacancies formed due to low oxygen partial pressures during heat-treatment. High dielectric constant values were achieved for 200nm thick films, so scaling of the dielectric to around 100nm should be possible. As the oxygen partial pressure during firing drops, the dielectric loss of Mn doped BaTiO3 film suddenly increased at low electric field. Carbon residue as well as oxygen partial pressure and composition can affect the dielectric properties. Removal of carbon residue was retarded from 750°C in air to 1000°C in nitrogen ambients. Also, residual carbon was found after 1000°C annealing in reducing ambient by electron energy loss spectroscopy (TEM-EELS) analysis. This residual carbon led to local reductions in the oxygen partial pressure during firing, reduced titanium ions, and to the presence of a Ni-Ba interfacial alloy layer on the surface of the Ni foils.