http://faq.ktug.org/faq/%B9%AE%BC%ADLayOut?action=show&redirect=LayOut

※geometry package를 쓰시던지

\usepackage[a4paper,left=40mm,right=35mm,top=30mm,bottom=25mm]{geometry}

※ oblivoir class에서 fapapersize package를 쓰세요. 

    \documentclass{oblivoir}
    \usepackage{fapapersize}
    \usefapapersize{210mm,297mm,40mm,35mm,30mm,25mm}
    \begin{document}

      blabla...

    \end{document}