Root [AT&T] How to recover space on the internal card

[Challenged_101]

First off, I don't do social media, not a gamer and hardly take any pictures. I've installed a dozen apps but most seldom get used.

The internal card shows almost 12GB used and 1.87GB free. I have a huge external card but that's not the point. The phone is rooted but running oem rom.

I want to restore some space before installing a custom rom. Misc.files/O is a whopping 7.67GB. I've opened each folder in attempt to identify the offender but can't find an option to reveal the volume in each folder.

I'm assuming I can safely delete the cache but is there any other options?

Thanks for any responses....

 

[OrionThaGreat]

What I would do is back up apps data and anything else you want to keep on to your external SD, the go in to recovery go into the mounts and storage option and erase everything except the external cuz it does give you the option. After flash your ROM again and the gapps then restore all your stuff if the problems continue I'd place a safe bet on it being your ROM.
*EDIT: Just to be clear go into recovery> Mounts and Storage> then format /boot, /system, /cache, /data, and /data and /data/media.
Don't forget to back up all your important data also make a backup of your current ROM only because if anything goes wrong which I doubt at least you'll have something to fall back on.