Can I reinstall macOS on my Mac after it's been turned into a Linux box?
Can I reinstall macOS on my Mac after it's been turned into a Linux box?


Of course you can, there's no Mac (or any computer for that matter) that has been irreparably limited to never running it's native operating system ever again. 

Yes, you can "brick" things but that's usually a firmware issue or in Apple's case, a security "thing" that you couldn't either unlock the firmware or the FileVault encryption.


  I have tried holding down the C-key on startup. I have tried resetting NVRAM.



Holding the C key tells the Mac to boot from the CD/DVD drive.  Did you have an install disk loaded?  If not, it's not going to do anything
The NVRAM is your pre-boot environment that sets things like which partition is your startup volume or what the screen resolution at boot should be.  Resetting your SMC/NVRAM is Internet folklore that it somehow fixes things when in reality, in 99.99% of the cases, it's not related to the problem at all.


The 2008 Mac Pro originally came with Leopard (10.5.x) but can run El Capitan (10.11.x).   The best way to get this installed would be to get ahold of an El Capitan USB Installer and install it that way...


have a friend with a Mac download and create the installer for you
take it to an Apple store and they will assist with the installation


When you get the installer created, simply put it into a USB slot, and hold the ⌥ Option key while booting.  This will take you into the boot manager where you can select the USB drive as the boot volume.