It's fairly easy to install your own fonts in Over, but there are a few caveats.
First of all... Before you install any font, make sure you have a license to use it on another computer/device.
Also, on Android devices, you'll need to first unlock the system fonts in FONT before you can install your own fonts for free. But once you've unlocked the system fonts, then have at it!
You can deliver .otf and .ttf font files to your device by email, dropbox or any other way that you can get a file to your device.
You must access a proper font file, .otf or .ttf (not ZIP files).
As an experiment, I asked someone else to send me a link to a font. On iOS, when I pressed the link, an option showed up in the upper right hand-side of the screen to "Open in Over" (or something similar).
You might have to experiment a bit with downloading fonts, and make sure they either .otf or .ttf. If you are trying to download fonts from free font sites, you will have a bit of difficulty. Most of those sites let you download a ZIP file and not the actual font file.
To delete your installed fonts on iOS, simply "swipe left" to bring up a "Delete" button.