1: press and hold together the Volume Down and Power key for a few seconds.
2: You have to let go of the Power key as soon as the Oppo logo appears on the screen.
3: Release the Vollume Down button as sonn as the Language Menu appears on the screen.
4: In this step select English using Volume keys to browse and Power rocker to select.
5: After that select Wipe data and cache option twice.
6: In order to confirm the whole process by selecting Yes.
7: Restart the cell phone by choosing the Reboot option.
more about here - http://www.hardreset.info/devices/oppo/oppo-r9/
