We investigated the effects of a high density $O_2$ plasma treatment on the structural and electrical properties of Ga-, B- codoped ZnO (GZOB) films. The GZOB films were deposited on polymer substrate without substrate heating by DC magnetron sputtering. Prior to the GZOB film growth, we treated a polymer substrate with highly dense inductively coupled oxygen plasma. The optical transmittance of the GZOB film, about 80 %, was maintained regardless of the plasma pre-treatment. The resistivity of the GZOB film on PC substrate decreased from 9.08 ${ imes}$ $10^{-3}$ ${Omega}-cm$ without an $O_2$ plasma pre-treatment to 2.12 ${ imes}$ $10^{-3}$ ${Omega}-cm$ with an $O_2$ plasma pre-treatment. And PES substrate decreased from 1.14 ${ imes}$ $10^{-2}$ ${Omega}-cm$ without an $O_2$ plasma pre-treatment to 6.13 ${ imes}$ $10^{-3}$ ${Omega}-cm$ with an $O_2$ plasma pre-treatment.