Here is what I know about this (used to work for a giant dealer)

The crank should be a standard triple bolt pattern, meaning the smallest granny would be 26T.

The rear der being a 105, I think even 32T is stretching the limit of that der quite a bit. Switch out the rear der for a LX or XT mountain, and it should be able to handle a 11-34 LX or XT rear cassette.

This would give you the same low gear ratio that most mountain bikes have, my MTB lowest is a 34 rear, 29 front.

Hope that helps.

Aside, why are you interested in the OCR? It is quite pricey for what you get, and I seriously reccomend you do NOT go with disc brakes on a touring bike. Not really neccessary, and they are easy to damage, and make racks and trailers hard to mount.

For similar money, you can get a Surly Long Haul Trucker outfitted with 105 or LX triple groups if you want a dedicated touring bike, or for more of a "light tour" look at either the Surly Crosscheck or Surly Pacer, both of which can be outfitted with a 105 triple group for around the same price, and be MUCH better and more reliable.

I personally own a Long Haul Trucker and a Crosscheck, and love them both.