Willl unlocking my Bootloader delete my OS I have decided to install Cyanogen Mod Android 5.1.1 onto my Xperia SP[C5302].

I am about to unlock the bootloader(the first step).

I have followed the steps and downloaded Android SDK(downloaded and installed google usb driver separately) and FastBoot driver.

Am about to perform the unlocking but am hesitant in doing so because am afraid whether unlocking my bootloader delete the OS. 

It's running on official 4.3 update from Sony and no custom works have been done ever since.

My question is whether unlocking bootloader will delete the os and I'll not be able to boot the phone again?
No. Unlocking your bootloader will not delete your os. It will delete your userdata which is not related to the /system partition. 

But always keep in mind that, especially with Sony devices, there are solutions without having you to unlock the bootloader.

Cheers