Your A83 sports a microSD memory card slot. If your laptop or personal computer is compatible with that card format (Most laptops are so-equipped), you should be able to transfer a music file
The A83 is a smartphone. Most smartphones should let you save any song found online by tapping and holding your finger on it and selecting 'save.' Once a music file is saved to your media library, touch-and-hold it and choose 'assign' from the drop-down menu.
The Oppo A83 is an Android smartphone and the Android market offers a bunch of free ringtone apps such as MP3 Ringtone Maker, RingDroid (open source) and more.
Here are some of the free ringtones (compatible with your Oppo A83) we offer:
Free Horror Ringtones Free Ghost Ringtones Free Ringtones-a-gogo Free Techno Ringtones Hear All Our Free Polyphonic Ringtones