Hi just to give a two pence worth - I do think there is a difference between applying to O and C in that, as goodbyestranger says, at O, decisions seem to be made far more at a departmental level. So in this sense, it doesn't matter which college you apply to as, if you're a strong enough candidate but perhaps not the strongest at that particular college in a given year, you will be interviewed elsewhere and perhaps again - ie. they will get in somewhere.
At C, the colleges seem to operate far more as separate entities. They say the pooling system means things shake out but I'm not sure about that as, if you look at some subjects at some colleges, they hardly ever take from the pool, whereas others seem to do so as a matter of course. I think it's largely down to the discretion of the interviewers / DOS as to whether they tend to take applicants they have actually met, or take their chances in one in the pool who they won't have met the applicant in person. They will just be 'a file' really, so harder to gauge.
So, I would say, if applying to C, college choice matters. It's not true that some colleges are 'harder to get into than others' as such, because it varies subject to subject. But do look at the interactive graphs for the last 5 years for the chosen subject at the different colleges. You can see not only which are more competitive, but also which seem to routinely take from the pool and which barely ever do.
Anothef thing to factor in at C is that some colleges in various subjects do a 'general interview' and an academic interview, whereas others will do two academic (and others perhaps just one longer academic). It really can vary a lot. If your DC has a very strong PS with things in it they feel they want to discuss, go for a college with a general interview as this will give them the opportunity to do so.
(sorry it was meant to be a two pence worth, but ended up like a mini essay)!