Pricing schemes for Oysters cards and Travel Cards are set up for commuters who usually go back and forth between the same two locations. Hence the confusion for tourists who wander around. (Typically, though, they usually stay in zones 1 and 2.)
Per the suggestion from ramblin' on, get an Oyster and put £30 on it. (I do this.) If you use that before the week is out, you can add money to it at the machines in the Tube stations. Unless you know exactly where you are going to go, and when, and on which portion of the transport network, you'll just get a headache trying to figure out the absolute cheapest thing to do.
The London Pass is a discount scheme for some of the usual tourist haunts. I've never used it. Its cost effectiveness would depend on how many of those sites you visit and what it costs to get in without the Pass.
The Advance train tickets are the cheapest. You can buy them online and choose to pick them up at the station or have them mailed to you. (At a station, you retrieve the ticket from one of the ticket machines, or stand in a queue and get them from a staff agent.)
Advance fares are for specific trains on specific days at a specific time. If that might be problematic, and you buy on the day of travel, going off-peak -- outside the rush hours -- will be cheaper. (Oxford is about one hour by train, Bath about two.)
