Why check tenant availability first

Every Microsoft 365 tenant gets a unique default domain in the form yourcompany.onmicrosoft.com. This name is reserved globally across all Microsoft 365 customers, so if another organization has already registered it, you won't be able to use it for a new Microsoft tenant, trial, or migration. Checking availability before you start setup avoids failed sign-ups and wasted time.

Step-by-step: checking with o365.rocks

  1. Open o365.rocks in your browser.
  2. Enter the tenant name you want to check (just the prefix, e.g. contoso for contoso.onmicrosoft.com).
  3. Submit the check. The tool queries Microsoft's public endpoints and reports back whether the name is:
    • Available — no Microsoft 365 tenant currently uses that name, so you're free to register it.
    • Taken — an existing Microsoft tenant already uses that name; you'll need to pick a different one.
  4. If the name is taken, try a variation (e.g. adding a region, department, or short suffix) and check again.

What to do next

  • If the name is available, proceed to create your new Microsoft 365 tenant using that name.
  • If the name is taken and you believe it should belong to your organization (e.g. a previous employee registered a trial), contact Microsoft support to investigate ownership before trying to reclaim it.
  • Always confirm the final tenant name with your IT team before rolling it out, since the default .onmicrosoft.com domain is difficult to change later.

Notes

  • This check only looks at the default .onmicrosoft.com name — it does not check custom domains (e.g. yourcompany.com) added to a Microsoft tenant.
  • o365.rocks is a third-party convenience tool; it reflects the same public availability signal Microsoft's own tenant sign-up page uses.

How D SCAPE can help with your Microsoft 365 tenant

Picking a tenant name is only the first step. As a Managed IT Service Provider (MSP) based in Bangkok, D SCAPE plans and runs full Microsoft 365 tenant setups for organizations across Thailand — from naming and licensing to security baselines and user migration. Our Microsoft 365 team can help you:

  • Design a tenant and domain structure that fits your organization (single tenant, multi-tenant, or a tenant-to-tenant migration).
  • Add and verify your custom domain, configure DNS, and set up mail flow correctly the first time.
  • Apply Microsoft 365 security best practices — conditional access, MFA, and data protection — from day one.
  • Manage ongoing licensing, cost optimization, and day-to-day Microsoft 365 administration.

See our Microsoft 365 service overview or talk to the D SCAPE team if you're planning a new Microsoft 365 tenant or migration.

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