I've heard of OVC but never used them. What they have done is combine 3 separate Carnival cruises of 17, 13, and 18 days into the 48 day package. There is no single 48 day cruise offered. Of course it would feel like one to you because you'd be doing a back-to back-to back. But you would be seeing two days of debarkation/embarkation for probably most of the passengers on the preceding legs. I would also brush up on my Spanish.
I suggest you look at what you can price out on your own and then add in your air and see how the deals compare. The first leg of 17 days goes from L.A. to Santiago. The 2nd cruise of 13 days goes from Santiago to Buenos Aires, and then the final 18 day cruise goes from there to NYC.
As much as I love cruising, I would go insane being on a cruise for that many days. The pricing for all three combined looks to be around $3500 per person for an inside cabin, so it is not an expensive cruise(s).