Two-dimensional (2D) tungsten selenide (WSe₂) is promising candidate material for future electronic applications, owing to its potential for ultimate device scaling. For improving the electronic performance of WSe₂-based field-effect transistors (FETs), the modification of surface properties is essential. In this study, the seamless structural phase transition in WSe₂ lattice is achieved by soft oxygen plasma, regulating the electrical conductance of WSe₂-based FETs. We found that during the soft oxygen plasma treatment with optimal processing time, the generated oxygen ions can substitute some selenium atoms and thus locally modify the bond length, inducing 2H → 1T phase transition in WSe₂ with seamless interfaces. The mosaic structures have been proven to tailor the electronic structure and increase the hole carrier concentration inside WSe₂, significantly increasing the channel conductance of WSe₂ FETs. With the further increase of the oxygen plasma treatment time, the creation of more selenium vacancy defects leads to the electronic doping, resulting in the reduction of conductance. Benefiting from the hexagonal boron nitride (h-BN) encapsulation to interrupt the partial structural relaxation from 1T to 2H phase, our WSe₂ FET exhibits high electronic stability with conductance of 6.8 × 10⁻⁴ S, which is about four orders of magnitude higher than 2H WSe₂ (5.8 × 10⁻⁸ S). This study could further broaden the WSe₂ FETs in applications for functionalization and integration in electronics.