Check the Domains FAQ if you don't find what you're looking for.
Sign in and click Settings > Domains > Buy a domain
Buy a domain from another domain registrar
If you want to buy a domain from a domain registrar other than GoDaddy, choose one that supports all DNS records needed by Office 365 services. For example, use one of the following registrars.
Transfer your domain to a different domain registrar
If your domain is managed by a provider that doesn’t support all the necessary DNS records, you can transfer it to a different registrar. When you transfer the domain, you change who you send payments to in order to renew and keep your domain name.
Request the transfer at the registrar that you want to move your domain to. Look on their website for an option such as Transfer DNS. Be aware that after they make the changes, it can take a few days update across the Internet.
How to buy a domain for Office 365 operated by 21Vianet
If you don’t already have your own domain, you can easily buy one online at a domain name registrar, domain reseller, or even at your current Internet provider. You get a domain name when you sign up for Office 365 operated by 21Vianet, for example, contoso.partner.onmschina.cn. But you may want to use a custom domain name, like fourthcoffee.com.
To set up a domain in Office 365, you must own a domain and change some of the DNS records for your domain.
Caution: Some domain registrars or DNS hosting providers do not allow creating all the DNS records required by Office 365. The following list of hosting providers supports all the needed records. If you’re thinking of using a different hosting provider, learn about potential service limitations.
After you register your domain (at a domain registrar), you sign in to Office 365 as an admin and set up your domain so you can use it with your email address and other services..
Important: China Internet compliance policy requires that you get an Internet Content Provider (ICP) number for your public website.
Note: The SharePoint Online Public Website information in this article only applies if your organization purchased Office 365 prior to March 9, 2015.