Pera, Franz and Heinz-Peter Schmiedebach. "O" Medizinischer Wortschatz. Berlin, Boston: Walter de Gruyter, 2007. 62-64. Web. Retrieved 26 May. 2013, from http://www.degruyter.com/view/books/9783110215137/9783110215137.62/9783110215137.62.xml

Pera, F. & Schmiedebach, H. (2007). O. Medizinischer Wortschatz (pp. 62-64). Berlin, Boston: Walter de Gruyter. Retrieved 26 May. 2013, from http://www.degruyter.com/view/books/9783110215137/9783110215137.62/9783110215137.62.xml

Pera, Franz and Heinz-Peter Schmiedebach. 2007. O. In Medizinischer Wortschatz, 62-64. Berlin, Boston: Walter de Gruyter. Retrieved 26 May. 2013, from http://www.degruyter.com/view/books/9783110215137/9783110215137.62/9783110215137.62.xml