OpenType fonts are installed the same way that fonts in other formats are installed. Fonts are installed in the global /Library/Fonts folder or the user's ~/Library/Fonts folder.
Fonts in the /System/Library/Fonts folder are located there by Apple because they are used by the OS. They are also available to third-party applications. Damaged or deleted System fonts may cause a world of hurt. Therefore, you should leave the /System/Library/Fonts folder alone.
Finally, there is absolutely no need to install any font in more than one location. Multiple copies of a font--even if they are in different formats--conflict with each other.