How to restore OS X on MacBook Air after installing Windows 8?
press the Option key + R
select the disk that installed
Mac OSx.
maybe you're starting with
Windows.

Now if you want to
a new installation, you will
need access to another Mac,
to mount the flash drive installation.
follow these steps: http://osxdaily.com/2015/09/30/create-os-x-el-capitan-boot-install-drive/