The OnePlus 5 is a smartphone. Most smartphones should let you save any music file you could find online by touching and holding your finger on it and selecting 'save.' Once a music file is added to your memory card, touch-and-hold it and choose 'assign' from the contextual menu.
The OnePlus 5 is an Android smartphone and the Android market offers a bunch of free ringtone apps such as MP3 Ringtone Maker, Myxer and more.
Here is a sample of the free ringtones (compatible with your OnePlus 5) we offer:
Free Horror Ringtones Free Ghost Ringtones Free Short Ringtones Free Techno Ringtones Hear All Our Free Polyphonic Ringtones