The new £320m rail line linking Oxford and London has opened in a deal between operator Chiltern Railways and Network Rail.
Chiltern Railways says it is the first train line to link a major British city to London in 100 years.
Two new stations in the county, Oxford Parkway and Bicester Village, will take passengers into London Marylebone.
Oxford Parkway is expected to attract an extra 250,000 return commuter journeys each year
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Prime Minister David Cameron, who is MP for Witney, said it was "an historic moment for Oxfordshire".
Commuters will be able to travel from the north of Oxford, just off the A34, near Water Eaton park-and-ride, to London Marylebone in under an hour. The direct line into the city of Oxford is expected to open in 2016.
Chiltern Railways
The rebuilt station for Bicester Village has also opened, directly linking the retail park to London.
The journey from Bicester Village to London Marylebone will take 46 minutes.
Oxford Parkway is expected to attract an additional 250,000 return commuter journeys each year.
The new service will offer two new fast trains per hour throughout the day between Oxford Parkway, Bicester Village Station and London.
Network Rail chief executive Mark Carne said the new line would help to boost capacity "as rail travel is forecast to double over the next two decades".
Mr Cameron said the new line meant "there will be more trains, faster journey times and better transport links for people living and working here".
