If you add in various 0th weeks, you end up spending pretty much exactly half the year at Oxford. Bear in mind that the huge long summer vacation is a big contributor to that - the Christmas and Easter breaks won't feel tremendously long.
If you're desperate to stay up, you can do open days\interview helping\college phone campaigns, or live out in your second\third year.
Oxford has a tendency to be a bit dead outside undergraduate terms, I'm finding.