There is one piece of clothing that is strongly associated with the city - Oxford bags. These are rather volumous trousers that were favoured by the upper class twits being educated in the city between the wars. At the same time the boys in Cambridge succeded in spliting the atom...spot the difference.
You can however buy youself an official sweathirt - and in the dark blue of Oxford they do (I will admit) look quite good.
The official ones are available in several shops around the city, including most obviously on Broad Street.
I've included as the picture a sweatshirt from a proper university
What to buy: Dark Blue Sweatshirt
What to pay: £ 30
Long established, Bead Games is the place for second hand clothing of character, components for beading, necklaces & Bric a brac of unusual appearance.
The owner tried to start a Rickshaw business in Oxford a few years back, but it never took off! Still, you can guess what to expect here from that!
What to buy: Best buy I had here was a necklace that looked like it was made of licorice allsort pieces which I used to wear with my "Life's a *** & so am I" tee shirt!
What to pay: Pot luck! Many bargains!