Thin films of $In_2O_3-SnO_2$(ITO), $SnO_2$, and $SiO_2$ were prepared on the PET substrate by DC magnetron roll sputtering. 135 nm thick ITO film on $SiO_2$/PET substrate has sheet resistance as low as 55 ${Omega}/square$ and transmittance as high as 85%. $H_2O$gas permeation through the film was 0.35 g/$m^2$ in a day. These properties are enough on optical film for the plastic LCD substrate or touch panel. Both refractive index and sheet resistance of ITO was found to be very sensitive to $O_2$ flow rate. Oxygen flow conditions have been optimized from 4 to 5 SCCM at $10^{-3}$torr. It is also shown that both thickness of $SnO_2$ and refractive index of $SiO_2$ decrease as $O_2$ flow rate increases.