Great choice for SB.
Carnival is a great line geared a lot more towards "younger" crowd, though they are trying to downplay that as much as possible. Its really family oriented too, and so you can pretty much expect the onboard events to be big, the deck party to be hopping, and expect a lot of people your age. Some of my best cruises have been on Carnival so far, and I can honestly say that I have never regretted a single moment.
O2 is in fact the teen club onboard, and yes, it is only for 15-17 (though exceptions are made from time to time). Activities are led by the teen staff, and participation is subjective to what the teens onboard feel like doing. My last cruise (on Carnival Liberty), we did some of the sponsored events, and we even added some events of our own (toga night). The great thing for teens is that Carnival has not, and never has had a curfew; of course, that doesn't mean they won't chase down rowdy people (we had a room blacklisted by security before). The other thing about Carnival is that I have felt like I have cruised solo with them whenever I've been on the line; it was complete independence.
If you have any further questions, please ask.