imac start at desktop and hang with spinning beach ball

Boot into Safe Mode. You should be able to clean up your desktop, then boot normally again. Note that everything goes a bit slower in Safe Mode, booting as well as certain routines in use.

Well, Safe Mode is slower, and the overload of files on the desktop slows the system further down. You can't relaunch the Finder in Safe Mode?

If that doesn't work, you need otherwise access to the folders and files on the HDD to restore or clean the desktop, and there are - alternatively - the following methods:

1.) Backup, preferably bootable (I hope you have a backup anyway...?).

2.) Connect your stuck Mac in Target Disk Mode to another Mac.

3.) Boot into Recovery System. If nothing else, from there you can re-install the system after wiping the disk.

Eventually 4.) in Safe Mode use a Terminal command to remove files from the desktop (but careful with Terminal commands - you gotta know exactly what you're doing there...).

By the way...

I even tried to reinstall OS X using Option+Command+P+R but after reinstall, the issue still occuring.

...booting with that key combination only resets the NVRAM (it does not re-install the OS), and that won't help, indeed.

I'd sure like to, but here it's now 1am, and that's a bit late to write a book or two... 😀

Perhaps someone else here can take over, or you use the search function here in the upper right corner of the forum pages, or Google. Here is e.g. Apple on Recovery System.