Contoso, Ltd. has an on-premises SharePoint environment. The company plans to deploy SharePoint Online.
You must use Active Directory Federation Services (AD FS). The global administrator account must be able to access the Office 365 tenant even if AD FS is unavailable.
You need to set up the global administrator account.
What should you do? tenant even if On-Premises ADFS environment is unavailable. You can provision new Cloud Account from the configuring-sharepoint-administrative-accounts-for-sharepoint-online/

A.
In the Office 365 admin center, create a user named sp_admin@contoso.onmicrosoft.com

B.
In the Office 365 admin center, create a user named sp_admin@contoso.com

C.
In Active Directory Domain Services Users and Computers, create a user named
sp_admin@contoso.onmicrosoft.com

D.
In Active Directory Domain Services Users and Computers, create a user named sp_admin@contoso.com

Explanation:

One of the first steps is to create SPO administrative account. You should always plan to create this account as
Cloud ID, E.g. sp_admin@yourdomain.onmicrosoft.com. Having this as Cloud ID, it allows you to access your
Office 365 Administration site.
References: https://nikpatel.net/2014/06/03/best-practices-for-configuring-sharepoint-online-tenant-part-ii-