Your Oppo R-11 features a microSD memory card slot. If your personal computer is compatible with that type of card (Most laptops are so-equipped), you should be able to save a music file
The Oppo R-11 is a smartphone. Most smart phones let you save any song found online by tapping and holding your finger on it, then selecting 'save.' Once the music file is saved to your media library, tap-and-hold it and select 'assign' from the drop-down menu.
The Oppo R11 is an Android phone and there are plenty of free ringtone apps such as MP3 Cutter, RingDroid (open source) and more.
Here are some of the free ringtones (Oppo R11-compatible) offered here at MartinsRingtones.com:
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