How To Hide Quick Launch Using CSS In SharePoint Online

There are three ways to hide quick launch in  SharePoint. 
  1. Using Out of Box
  2. Using SharePoint Designer
  3. Using CSS 
Using Out of Box
Go to Site Settings ->  Navigation Elements, and uncheck the "Enable Quick Launch" box.

Using SharePoint Designer

Open SharePoint Designer, click on Home, and disable the "Quick Launch" checkbox.

Using CSS
Add Webpart->Media and Content-> Add Script Editor Webpart.

Edit the Webpart -> Edit Snipped -> Embed the code over here.

  1. <style type="text/css">  
  2.  #sideNavBox {DISPLAY: none}  
  3.  #contentBox {MARGIN-LEFT: 5px}  
  4. </style>  
This is the final result.

Please try it, and let me know if you are having any problems.