Installed Windows on Mac, now want to go back to OS X
Reinstall macOS (OS X)


Boot to  Internet Recovery. See instructions below. You may need to reset your NVRAM. If so, see instructions below.
Use the Disk Utility app to erase the internal drive. Choose for Mac OS Extended (Journaled) for the format.
Install macOS (OS X).
Boot to macOS (OS X).
If the operating system is Snow Leopard and the version is not 10.6.8, then download and install this Mac OS X 10.6.8 Update Combo v1.1.
If the operation system is 10.6.8, then install this Mac App Store Update for OS X Snow Leopard
Upgrade to 10.11.6.


Start up from macOS (OS X) Recovery over the Internet


Turn on or restart your Mac, then immediately hold down the option+⌘+R key combination.
Release the keys when you see the Apple logo or a spinning globe.
When you see the macOS (OS X) Utilities window, you have finished booting.


Reset your computer’s NVRAM or PRAM


Shutdown the Mac.
Locate the following keys on the keyboard: option, ⌘, P, and R. You will need to press and hold these keys simultaneously in step 4.
Turn on your Mac.
Immediately press and hold the option+⌘+P+R key combination, before the black screen with the Apple logo appears.
Continue pressing the keys until your Mac restarts, about 15 seconds.
Release the keys.


Appendix

How to reset NVRAM on your Mac

How to reinstall macOS

About macOS Recovery