.ONECACHE File

File extension onecache is exclusively related to Microsoft OneNote, a easy-to-use note-taking and information-management program in the  Microsoft Office system with which you can capture your thoughts and ideas in electronic notebooks, where you can easily organize, search, and share them.

A .onecache files are used for caching.

How to open:

It does not seem to be possible to manually access the contents of .onecache files.

How to convert:

Temporary files usually cannot be converted to anything else.