Are you saying you wish to travel:
Day 1: Oxford - Swansea
Day 2(?): Swansea - Manchester
Day 3(?): Manchester - Oxford
It's difficult to see how the return portion is going to be much use, so I suggest swapping the Off Peak Return for an Off Peak or Super Off Peak Single (this may incur a £10 admin fee, but if you bought it from the same ticket office you take it back to, there is a possibility they might waive it)
If you can commit to specific trains, you'd be best off with Advance fares for Swansea - Manchester and Manchester - Oxford.
If you need flexible tickets, then we could have a go at providing some suggestions but I'd want a rough idea of your planned itinerary before doing that.