There are two separate bus companies operating in Oxford, which for a place of this size can be confusing as often different buses compete for fares along the same route.  There are daily tickets and even an interchangeable (for both operators) one.  What may be convenient for you will depend on which part of the city you will be staying, so you should find this out with your hotel. Generally speaking you shouldn’t need to worry about public transport as the main sites are all within walking distance, though it can be a long walk if you are going from University park to St Aldates.