The OST to Office 365 Challenge
OST files cannot be imported directly into Microsoft 365. Unlike PST files, which Microsoft’s import tools accept natively, OST files are tied to specific Outlook profiles and must be converted before they can be migrated to the cloud.
The standard approach: convert OST to PST first, then import the PST into Microsoft 365.
Comparison Table
| Tool | Approach | Platform | GDPR | Price | Best For |
|---|---|---|---|---|---|
| MailtoPst | OST → PST online | Any browser | Yes (EU) | Freemium | Most users |
| Stellar OST Converter | OST → PST desktop | Windows | No | ~$49-149 | Corrupted OST repair |
| Kernel OST to PST | OST → PST desktop | Windows | No | ~$39-149 | PDF export |
| SysTools OST Recovery | OST → PST desktop | Windows | No | ~$49-299 | Advanced filtering |
| Remo OST to PST | OST → PST desktop | Windows | No | ~$80-170 | Basic conversion |
MailtoPst
Convert OST to PST online without installing any software.
Pros: Works on any platform, GDPR-compliant EU servers, automatic file deletion, no installation needed, supports additional formats (EML, MBOX, MSG).
Cons: Requires internet, upload time for large files.
Workflow: Upload OST → Download PST → Import to Microsoft 365 via network upload or Outlook.
Stellar OST Converter
Desktop tool with built-in repair capabilities for corrupted OST files.
Pros: Handles corrupted files well, preview before conversion, established brand.
Cons: Windows only, higher price, no GDPR compliance.
Kernel OST to PST
Desktop converter with PDF export option.
Pros: PDF export, date filtering, handles both ANSI and Unicode.
Cons: Windows only, limited free version (25 items per folder).
SysTools OST Recovery
Enterprise-oriented desktop tool with advanced filtering options.
Pros: Granular filtering, batch processing, date range selection.
Cons: Windows only, complex pricing tiers, steep learning curve.
The Complete Migration Workflow
Regardless of which OST conversion tool you choose, the migration to Microsoft 365 follows these steps:
- Convert OST to PST — Use MailtoPst or a desktop tool
- Verify the PST — Open in Outlook to confirm folder structure and content
- Import to Microsoft 365 — Use Outlook desktop import (small files) or network upload (bulk)
- Verify in cloud — Confirm messages, folders, and attachments arrived correctly
- Clean up — Archive or delete the source OST and intermediate PST files
Verdict
MailtoPst is the best starting point for most OST to Office 365 migrations. Its browser-based approach works on any platform, GDPR compliance provides documented security, and the conversion quality matches desktop tools.
For severely corrupted OST files, Stellar’s repair capabilities may be worth the Windows-only constraint. For enterprise migrations with advanced filtering needs, SysTools offers more granular control.
Convert OST to PST with MailtoPst →
FAQ
Can I import OST directly to Office 365?
No. OST files are cache files tied to specific Outlook profiles and cannot be imported directly. You must first convert OST to PST, then import the PST into Microsoft 365.
What if my OST file is from a deleted Exchange account?
This is exactly the scenario where OST conversion is essential. The account is gone, but the OST file on the workstation still contains the cached mailbox data. Convert it to PST to recover the data, then import to the new Microsoft 365 account.
How large can an OST file be for conversion?
MailtoPst supports OST files up to 50 GB on Enterprise plans. Desktop tools generally have no hard limit but may slow down with files over 20-30 GB.