We report on the influence of oxygen flow rate on structure, composition, density, deposition stress and optical properties of the as-deposited tantalum oxide thin films. The films were prepared by reactive direct current magnetron sputtering. The sputter current and total pressure were kept constant at 300 mA and 0.8 Pa, respectively. We could deposit fully transparent films at a rate of approximatey 6 nm/min. without noticeable substrate warming from the plasma. Grazing angle XRD showed the films to be amorphous at all oxygen flow rates. Simulations to RBS data revealed, within errors, stoichiometric films above 2 sccm oxygen flow. Moreover argon incorporation in the films above 2 sccm oxygen flow was noted. The density was found to steeply decrease upto 2 sccm followed by a very slow linear decrease with oxygen flow as deduced from X-ray reflectometry. The refractive index, the extinction coefficient and the band gap energy were all obtained by optical spectroscopy. A band gap which increased from 4.17 to 4.23 eV with oxygen flow was determined for films in the transparent region. A characteristic of the defects in the film, γ, which is obtained by simulating the optical spectra, was found to decrease from 85 meV at 6 sccm to 60 meV at 15 sccm oxygen flow. There was no significant change in γ above 15 sccm. On the other hand the refractive index and the extinction coefficient were found to slightly decrease with increasing oxygen flow for the transparent films. Stress data revealed the films to be under some compressive stress upon deposition. The stress decreased with increasing oxygen flow and stabilized at roughly –250 MPa above 6 sccm oxygen flow.