Some good advice here - I agree with those who suggest that you not buy a pack just for your OLS session. Either borrow a pack, or use something you already have or is inexpensive until you have a bit more experience and/or until you have a better handle on what else you'll do with it.
Like others who have been doing this a while, I have several packs. For monthly campouts, OA campouts and training that involves camping, I use a two-strap military duffle because I take way more stuff on a local campout than on a backcountry trip - I often take multiple of the same item so I can test new stuff. I have a canoe portage pack for those trips and a 72L Golite pack that I absolutely love for backpacking.