How to install OS X on a Windows only Mac?
Turn your Mac off.
Turn it on, but hold down the Option key until you see images of two hard drives on screen.
Use the arrow keys to select "Recovery", press enter when you have selected it.


If you wish to erase Windows and your Windows data, if not, skip to step 9:


Click Disk Utility.
Click the partition under whatever your internal SSD is called, typically the brand name followed by some seemingly meaningless characters.
Select Erase in the toolbar
Confirm you wish to erase the drive
Upon completion of the wipe of your drive, close the window.
Select Install OS X and follow the on-screen instructions.