As an ex local this would be the walking route I’d do. If coming M40 then A34 I’d use Oxford Parkway park and ride (frequent buses). I’d recommend getting off at the stop just before St Giles (opposite Little Clarendon Street). Walk down Keble Road, right along Parks Road (Pitts Rivers / University Museum). I’d recommend The Kings Arms on the corner (best to sit in the snug at the back for atmosphere) to Catte Street (Bridge of Sighs), down Queen’s Lane to the High Street, across to Magpie Lane, right along Merton Street back to the High St, right then left past St Mary’s church, left down Brasenose Lane to covered market, out along Cornmarket towards The Ashmolean Museum and back to St Giles. Sounds complicated but have a look at Google Maps. Takes maybe an hour normal walking speed. You’ll see a lot of recognizable places. Oxford city centre really isn’t that big. Have a great day.