A 7-mile long £75m rail link has opened that joins Surrey Quays in the south-east with Clapham Junction in south-west London.
The orbital, which London Mayor Boris Johnson called the "M25 of rail", will mean cheaper fares and cut congestion by avoiding central London, according to Transport for London.
But critics feel those bound for central London will be "worse off" as they will lose the links which were earlier provided by the South London Line.
It takes over South London Line services between London Bridge and Victoria via Denmark Hill and Peckham Rye. The service was withdrawn as Thameslink undergoes an upgrade.
The Overground now goes through 21 of London's 32 boroughs, and the orbital will allow passengers to go on its different routes via Willesden Junction, Clapham Junction, Highbury and Islington and Gospel Oak stations.
Four trains an hour will run on the route, stopping at Queens Road Peckham, Peckham Rye, Denmark Hill, Clapham High Street and Wandsworth Road, carrying up to 12.3 million people a year.
The new stations connecting Surrey Quays to Clapham Junction are:
Queens Road Peckham
Clapham High Street
This is the final stage of a £1.5 billion investment to improve the London Overground.
Now the extension between Surrey Quays and Clapham Junction is open, the Highbury & Islington to West Croydon/Clapham Junction line will have trains running up to every 15 minutes between:
Highbury & Islington and Clapham Junction
Highbury & Islington and Crystal Palace
Dalston Junction and New Cross
Dalston Junction and West Croydon
This means that trains will run every 3 to 5 minutes between Dalston Junction and Surrey Quays.