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OST Office 365

Convert OST to Office 365
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Migrate OST files to Office 365 (Microsoft 365). Convert offline Outlook cache to cloud mailbox. Step-by-step guide with multiple methods.

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What is a OST file?

OST (Offline Storage Table) is the local cache file created by Microsoft Outlook when connected to an Exchange Server or Office 365 account. Unlike PST files which are standalone archives, OST files are tied to a specific Outlook profile and Exchange account.

OST files are created automatically when:
- Outlook connects to Exchange — Outlook caches a local copy of your mailbox for offline access
- Cached Exchange Mode is enabled — the default setting in most Outlook configurations
- Office 365 accounts are configured — Outlook creates an OST for each connected account

The challenge with OST files is that they cannot be opened directly outside the Outlook profile that created them. If the Exchange account is decommissioned, the user's profile is deleted, or the server is no longer accessible, the OST file becomes orphaned. This is why converting OST to a portable format before importing to Office 365 is often necessary.

What is a Office 365 file?

Office 365 (Microsoft 365) provides cloud-hosted Exchange Online mailboxes that replace the need for local OST files. When you migrate OST data to Office 365, you are effectively restoring orphaned or archived Outlook cache data into a live cloud mailbox.

After migration to Office 365:
- Emails are accessible everywhere — Outlook desktop, web, and mobile apps all sync automatically
- No more orphaned data — emails live in the cloud, not in local cache files
- Built-in redundancy — Microsoft replicates data across multiple data centers
- Compliance features — retention policies, eDiscovery, and legal holds are available
- Seamless collaboration — shared mailboxes, calendars, and contacts across your organization

The migration process typically involves converting the OST to an intermediate format (PST, EML, or MBOX) before importing into the Office 365 mailbox.

Why convert OST to Office 365?

Migrating OST files to Office 365 addresses several critical business scenarios:

1. Recovering data from orphaned OST files
When an Exchange server is decommissioned or a user profile is corrupted, OST files become inaccessible through normal means. Converting and importing them to Office 365 recovers valuable email data that would otherwise be lost.

2. Migrating from on-premises Exchange to Exchange Online
During an Exchange hybrid migration, some mailbox data may exist only in local OST cache files. Extracting and importing this data ensures nothing is left behind during the cloud transition.

3. Employee offboarding and archival
When employees leave, their OST files on company laptops contain email data that may be needed for compliance, legal holds, or knowledge transfer. Importing to Office 365 centralizes this data.

4. Disaster recovery
If an Exchange server fails catastrophically, OST files on user workstations may be the only copy of recent emails. Converting and uploading them to Office 365 is a critical recovery step.

OST vs Office 365 — Key Differences

Feature OST Office 365
File type Local cache file (profile-locked) Cloud-hosted mailbox
Portability Tied to single Outlook profile Accessible from any device
Data safety Local disk only (no redundancy) Geo-redundant cloud storage
Compliance No built-in tools eDiscovery, retention, legal holds
Accessibility Requires original Outlook profile Web, desktop, and mobile apps
Backup Manual (user managed) Automatic (Microsoft managed)

3 methods to convert OST to Office 365

Method 1: Reconnect Outlook profile and sync

If the original Exchange account still exists, simply reconfigure the Outlook profile to connect to Office 365. Outlook will sync the cached OST data to the cloud mailbox automatically.

Pros

  • ✓ Seamless if account is active
  • ✓ Automatic sync, no manual steps
  • ✓ Preserves all data and structure

Cons

  • ✗ Only works if Exchange account exists
  • ✗ Cannot recover orphaned OST files
  • ✗ Requires the original Outlook profile

Method 2: Convert OST to PST, then import to Office 365

First convert the OST file to PST format using MailtoPst or a similar tool. Then use the Microsoft 365 Import Service or Outlook desktop to import the PST into your Office 365 mailbox.

Pros

  • ✓ Works with orphaned OST files
  • ✓ PST is the standard import format for Office 365
  • ✓ No original Exchange account needed

Cons

  • ✗ Two-step process
  • ✗ Requires conversion tool
  • ✗ Large files take time to process
Recommended

Method 3: Convert OST with MailtoPst (online)

Upload your OST file to MailtoPst and convert it to EML, MBOX, or PST format. Download the converted files and import them into Office 365 through your preferred method. No Outlook installation required.

Pros

  • ✓ Works from any browser, any OS
  • ✓ No Outlook or Exchange needed
  • ✓ Handles orphaned OST files
  • ✓ GDPR compliant, EU servers

Cons

  • ✗ Requires internet connection
  • ✗ Multi-step process
  • ✗ Paid after first conversion
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Step by step

How to convert OST to Office 365 with MailtoPst

01

Upload your OST file

Drag & drop or click to upload your .ost file. Files up to 51 GB supported with resumable upload.

02

Choose Office 365 as output

Select Office 365 from the format dropdown. MailtoPst auto-detects the best conversion settings.

03

Download your Office 365 file

Conversion takes seconds. Download your .office365 file or receive it by email.

Convert your OST to Office 365 now →

100 MB free credit. No credit card required.

FAQ

Frequently Asked Questions

Can I directly import an OST file to Office 365?

No. Office 365's Import Service only accepts PST files. You need to convert the OST to PST first using a tool like MailtoPst, then import the resulting PST into your Office 365 mailbox.

My Exchange server was decommissioned. Can I still recover my OST?

Yes. Even though the OST is orphaned, tools like MailtoPst can read the file directly without needing the original Exchange server. Upload the OST, convert it to PST or EML, then import the results into Office 365.

Will calendar and contacts from my OST be imported to Office 365?

It depends on the conversion method. When converting OST to PST, calendar entries and contacts are typically preserved. When converting to EML or MBOX, only email messages are included. For a complete migration, the OST-to-PST path is recommended.

How large can my OST file be for migration?

MailtoPst supports OST files up to 50 GB. For the Microsoft Import Service, PST files up to 20 GB are recommended for best performance. If your converted PST exceeds 20 GB, consider splitting it into smaller files.

Do I need Outlook installed to convert OST to Office 365?

No. MailtoPst processes OST files server-side in the cloud. You only need a web browser to upload the file and download the converted output. No Outlook, Exchange, or Windows required.

Related conversions

OST → EML OST → MBOX OST → MSG OST → PST PST → Office 365
See all 30 conversion paths →

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