The 2.4km extension has four new stations at Westwood, Oldham King Street, Oldham Central, and Oldham Mumps.
Services run at 12-minute intervals throughout the day, with a journey time of around 30 minutes between Oldham Mumps and Victoria.
With the opening of the town centre line, the section of the former heavy rail line between Werneth and Oldham Mumps, which has been used by trams on an interim basis since the launch of light rail services to Oldham Mumps in 2012, will be closed.
The opening of the Oldham Town Centre extension takes the total length of the Metrolink network to 77km.
Testing is underway on the final section of the route between Rochdale mainline station and Rochdale town centre, which is due to open in the spring.
