If you are just doing day trips from Oxford, no way are you going to spend that much on rail fares in 15 days.
For example off-peak return fares:
Oxford-London �20 each including unlimited tube/bus rides
Oxford-Hereford �16 each
Oxford-Birmingham �24.90 each
Oxford-Southampton �22 each
Oxford-Bristol �15 each
There are reductions for some of these destinations if you book specific trains in advance.
You'd be better off getting senior rail cards for �20 each (can be bought on the spot at any manned station - just fill in a form and show your passport or other proof of age) and you'll get a third off the fares I've quoted and most others. Only restriction is that in the London area you can't use them untiol after the morning rush hour
Some trips are better done by bus - eg Oxford-Cambridge, maybe Oxford-London