I used to live in Bath, if you're coming from the airport you'll probably want the Air Decker bus. The bus is pretty regular and the drivers are usually happy to let you off at any stop, it'll take you right into the centre of Bath and you can grab a taxi/train from the nearby station to Oldfield Park which has its own train station. I'd recommend buying your Air Decker tickets in advance if you do, it's cheaper and easier