This info is in the instruction guide -
http://www.cic.gc.ca/english/information/applications/guides/5609ETOC.asp
Immigration Office requested for the processing of your application
Indicate the name of the Immigration Office which serves:
your country of nationality, or
the country in which you are residing, provided that you have been lawfully admitted to that country for at least one year.
Note: Do not select the CIO as the immigration office requested to process your application. Although you may be required to submit your application to the CIO, you must indicate the name of an appropriate visa office using the criteria above.
If you have been lawfully admitted to Canada for at least one year, you can choose CPP-Ottawa, or you can chose the Visa office serving your country.