A thermosetting oligomer or thermosetting polymer is provided. The thermosetting oligomer or thermosetting polymer contains repeating units, each of which has at least one thermosetting functional group in the side chain and is represented by Formula 1: where repeating units include X1, A1, and Y1 subunits, sidechain units include linking unit L and thermosetting functional group Z, and n is an integer from 1 to 4. The thermosetting oligomer or thermosetting polymer has a low coefficient of thermal expansion and high or no glass transition temperature, stiffness, processability, heat resistance and mechanical properties. The thermosetting oligomer or thermosetting polymer is highly soluble, wettable and dimensionally stable and is suitable for use in films, prepregs and printed circuit boards. Further provided are a thermosetting resin composition including the thermosetting oligomer or thermosetting polymer and a printed circuit board using the composition.