Ta$_3$N$_5$ has aroused interest in the scientific community as a small-band gap semiconductor able to split H 2 O molecules under sunlight illumination. Although its band gap of 2.1eV is highly suited, its synthesis is difficult. In this work, DC magnetron sputtering is used to successfully deposited thin films of Ta$_3$N$_5$ : the influence of the working gas composition Ar/N$_2$ /O$_2$ on the film properties is studied, notably the role of oxygen as a crucial ingredient for the structure. Previous attempts using magnetrons employed RF sputtering and investigated primarily temperature effects. The films were characterized structurally, optically (transmission/reflection spectrophotometry), electrically, chemically and subjected to photocatalytic measurements in 0.1M K$_2$ SO$_4$ aqueous solution adjusted to ph=11 by KOH. We show that variations scale with the O$_2$ flowrate: from amorphous at high O$_2$ flowrates, to Ta$_3$N$_5$ crystallization at reduced O$_2$ flowrate. Photo-spectrometry analyzed with a Tauc-Lorentz oscillator model shows that the band gap scales with the oxygen flowrate, from a metallic behavior to a maximum gap of 2.5 eV. Photocurrent measurements corrected by the conductivity in darkness show an insulating character for all films. Nevertheless the photo-conductivity drops by 9 orders of magnitude with increasing O$_2$ flowrate to reach minimum values around 10$^{-9}$ S/m..