There is a National Express coach, but it doesn't go anywhere near as frequently as the Oxford Tube. Takes around 2hrs to Victoria.
There are two main train routes - the one to King's Cross which takes about 50 mins, and the other to Liverpool Street which takes about 1.5 hours (longer if it's a stopping service). The Liverpool Street one is a bit cheaper.
London is pretty easy to get to in comparison with lots of other places in the country, but I wouldn't plan on going home too frequently in term time, you'll just be too busy!