Q: 1/3 of iMac HD taken by "Other"; How to find & delete?

Other=Email, Contacts, Calendars, OS X and other things not in one of the categories with names.

Be careful, and identify before you delete

The other includes the following which is from this Apple document: OS X: What is "other" space in About This Mac? - Apple Support

• Items in the OS X folders such as the System folder and caches
• Personal information such as documents, contacts, calendar data
• App Plugins or extensions
• Media files that cannot be classified by Spotlight as a media file because they are located inside of a package
• File types not recognized by Spotlight

So you'll have to peruse your documents, contacts, etc. and determine which you can delete.  One way to clear up space easily and quickly is to get an external HD and move your iTunes and Photos libraries to it and run them from there.  Be sure they work as expected before deleting from the boot drive.

OS X: What is "other" space in About This Mac? - Apple Support

Sorry OT, was right behind you with this.

Nobody said it's normal, but I said be careful what you delete, files categorized other are not identified and some are not unwanted.

Have you tried just restarting your Mac? Sometimes that can reduce "Other".

Re-index Spotlight and see what the results are.

Rebuild the Spotlight index on your Mac - Apple Support

First you must identify, then you can delete.

Right now you have no idea what it is, only that it is not Audio, Apps, Photos, Movies or any of the other named categories

It's process of elimination, you know what it is not, so look through those and don't delete the ones you want. If you are looking for a quick way you're out of luck.

Try running OmniDiskSweeper as root. Assuming it's in /Applications…

Copy and paste this command in the Terminal

sudo /Applications/OmniDiskSweeper.app/Contents/MacOS/OmniDiskSweeper

See if there is something in the hidden folders.