In an oral thesis defense, examiners often ask questions related to their own specific fields of study. Before your defense, take some time to become familiar with the particular areas of research interest held by the members of your examining committee. Write down some of the questions you think they might ask and formulate your answers. Then get together with a fellow graduate student. Ask each other the questions you think the examiners will ask, and practice your answers until you feel comfortable with them.
Some graduate students have found it useful to meet their examiners beforehand so they can get acquainted and learn more about the questions examiners may ask.