Leave Milton Park and head toward Oxford city centre to begin the walking loop; aim to arrive in central Oxford about 30 minutes later. This positions you to start at the High Street/Ashmolean area.
The University of Oxford's museum of art and archaeology with collection highlights from ancient Egypt to modern art; it's free and gives a quick, high-quality cultural overview. Open broadly 10:00–17:00 (confirm any late changes or special-exhibition fees).
See the iconic Radcliffe Camera from the outside and visit the University Church for panoramic views from the tower if open; the tower normally opens until mid/late afternoon so this early slot is ideal for photos and a short climb. Tower opening commonly until ~17:00 but check on the day.
Walk through the historic Bodleian area—Old Schools Quadrangle and the visitor centre—where you can admire medieval architecture; note that formal Bodleian guided tours and Divinity School entries may require prior booking and have set times.
Visit one of Oxford's largest and most famous colleges—see Tom Quad, Christ Church Cathedral and the dining hall (inclusion subject to college opening hours); Christ Church is open to visitors typically until about 17:00, so this mid-afternoon visit fits well.
Stroll east along the High to photograph the Hertford Bridge (aka Bridge of Sighs), the Clarendon Building and the Sheldonian Theatre; these are excellent quick stops for architecture and student-life scenes, all exterior and freely viewable.
Walk to Magdalen College to view its beautiful chapel, deer park and riverside setting; college visitor access typically starts in the afternoon (often from ~14:00) and closes around 17:00, so this mid/late-afternoon slot usually works well.
Take a relaxing walk through Christ Church Meadow or along the Thames/Isis towpath to enjoy riverside views; punting is a classic Oxford option if you want to replace the walk (punting operators often run into the early evening—check exact closing times).
Leave central Oxford to head back toward Didcot rail station so you can arrive roughly 18:00–18:30 depending on traffic; this ensures you meet your 6–7pm return window.