A 5,000 tonne bridge deck was slid into the place on the new A34 Wolvercote Viaduct this weekend.
The work was part of a £44m project to replace the bridge, which was built in the 1960s, with a new structure designed to last 120 years.
The bridge slide was part of a construction technique which allowed the Highways Agency to demolish and rebuild the bridge.
About 70,000 vehicles use the road every day.
Highways Agency project manager Ian Johnson said: "We are delighted that the bridge slide went so well. It was a massive job, and we are well on the way to finishing building the brand new structure, which will last well into the 22nd century.
"Our engineers have worked extremely hard throughout this project to keep delays to road users to an absolute minimum and keep traffic flowing around Oxford.
"With the bridge slide complete, it should only be a matter of weeks before traffic on the A34 is back to normal. The whole project should be finished in good time for this autumn."
The bridge slide, completed overnight on Saturday, involved moving the southbound bridge deck, which weighs 5,000 tonnes and is 250 metres long, 16 metres from its current position alongside the A34 to alongside the northbound carriageway, which was finished last year.
The Wolvercote Viaduct carries the A34 between the Midlands and the South Coast over the Oxford to Birmingham railway line, the Oxford Canal and the A40.
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