Yes, you do need a study permit (if the length of the program is greater than 6 months). The open work permit only permits you to work, not study. For a study permit, you need to show proof you're registered for the course. There is no "open" study permit option, like the open work permit.
This is from the boilerplate approval in principle letter out of IP8:
http://www.cic.gc.ca/english/resources/manuals/ip/ip08-eng.pdf
"If you are not already in possession of a valid work permit or you have a work permit but wish to
work elsewhere, or do not possess a valid study permit and you wish to attend school for more
than six months, you may apply for either or both.
If you wish to apply for a study permit, be sure to include a letter from the educational institution
you plan to attend. The letter should outline the type of course or program you are registered for,
the start date and the expected completion date."