Recently, π-conjugated polymers (CPs) applied to graphitic carbon nitride (g-C3N4) with high crystallinity have received great attention for removing organic pollutants. Herein, we propose that doping oxygen atoms into the g-C3N4 skeleton can improve photodegradation efficiency due to excellent carrier separation through enhanced polarization promoted by π-conjugated system expansion and high crystallinity. In detail, the higher electronegativity and larger size of oxygen atoms can enhance the crystallinity and conjugated plane of g-C3N4 leading to an increase in electrical conductivity and electron delocalization of O-doped g-C3N4, which extremely enhances the polarization degree. Therefore, the mechanism of the improved photocatalytic performance for diclofenac sodium, bisphenol A, and phenol can be attributed to the rapid photodegradation caused by fast charge separation. We believe that this paper can offer a novel perspective on the relationship between crystallinity, conjugation degree, and polarization in photocatalysts for environmental purification.