Now in an on premise CRM system the organization name can be found from the URL or from the deployment manager. But there's no such flexibility in case of CRM Online.
To get the organization unique name you need to go to
Settings -> Customization -> Developer Resources and there you would find the name of the organization as you can see in the below screenshot.
This is the organization name you can use to connect to your CRM Online instance with utilities such as Web Resource Upload Utility :)