Why does anyone ever go on these, especially in a small town like Oxford?
Fun Alternatives: The best way to see Oxford is by walking - either bring along your own guidebook or go on one of the walking tours which are cheaper and probably more fun.
Some wouldn't call this a tourist trap, but in my opinion a bus tour through a city centre which is easily walkable within 20 minutes doesn't make sense at all. It's the same as everywhere: You can see the colleges from aboard the bus, but you can't get in. The bus doesn't stop when you have discovered the number one photo stop. The commentary makes it hard to concentrate on seeing the colleges as well as listening to their history. And so on, and so forth...
The biggest drawback, however, is that you just drive by the buildings from the outside. Apart from some gargoyles and grotesques that you can spot from the bus, all the interesting things are INSIDE the colleges. You won't be able to see Exeter's magnificent Fellow's Garden, Radcliffe Camera surrounded by some of Oxford's most beautiful buildings, Christchurch's great hall or the fritillaries blooming in the grounds of Magdalen. You will see walls. That's it.
Unique Suggestions: Hop-on-hop-off-tours may be useful if you are in the town for 2 hours and want to get a first glimpse of its beauty before coming back. But coming back is essential, otherwise you won't see the real Oxford.
Fun Alternatives: Walking around! Oxford's city centre with 90% of its most popular sights is easily accessible by foot. It's not even a 10 minutes walk from Magdalen College to Carfax Tower, it's 5 minutes from Exeter to Christchurch, it's 5 minutes from the Ashmolean Museum to the Sheldonian Theatre. Everybody can do that by foot!