Two Outlook Formats, Two Different Purposes
Both PST and OST are file formats created by Microsoft Outlook, but they serve fundamentally different roles. Understanding the distinction is critical when you need to recover emails, migrate data, or troubleshoot Outlook issues.
PST (Personal Storage Table) is a portable archive file. You create it, you control it, and you can move it to any computer with Outlook installed.
OST (Offline Storage Table) is an automatic local cache. Outlook creates it behind the scenes to keep a synchronized copy of your server-based mailbox so you can work offline.
Side-by-Side Comparison
| Feature | PST | OST |
|---|---|---|
| Purpose | Portable email archive | Local cache of server mailbox |
| Created by | User or admin (manual export/archive) | Outlook automatically |
| Portability | Can be opened on any Outlook installation | Locked to the Outlook profile that created it |
| Server connection | Not required | Requires active Exchange/M365 account |
| Contains | Emails, contacts, calendar, tasks, notes | Mirror of server mailbox content |
| Typical use | Backup, migration, archiving | Offline access to Exchange/M365 |
| Can be shared | Yes (copy the file to another machine) | No (profile-locked) |
| Default location | Documents\Outlook Files\ | AppData\Local\Microsoft\Outlook\ |
When Outlook Creates Each Format
PST Files Are Created When You:
- Choose File, then Info, then Account Settings, then Data Files, then Add in Outlook.
- Use the Import/Export wizard to export your mailbox.
- Enable AutoArchive, which moves old messages to a PST file on a schedule.
- Manually drag messages to a PST data file in the folder list.
OST Files Are Created When You:
- Add an Exchange, Microsoft 365, or Outlook.com account to Outlook.
- Outlook automatically creates the OST to enable Cached Exchange Mode.
- You have no direct control over when or where the OST is created (though you can change the location in advanced settings).
The Portability Problem
The most important practical difference is portability. If you copy a PST file to a USB drive and plug it into another computer running Outlook, you can open it immediately. PST files are self-contained archives designed to be moved.
OST files, on the other hand, are bound to the Outlook profile and Exchange account that created them. If you copy an OST to another machine, Outlook refuses to open it. Even on the same machine, if the Outlook profile is deleted or the Exchange account is decommissioned, the OST becomes inaccessible through Outlook.
This is why converting OST to PST is one of the most common email recovery operations. It transforms a locked, profile-bound cache into a portable archive. See OST to PST conversion.
When You Might Need to Convert
- Employee leaves the company — their Exchange account is disabled, but the OST on their laptop has critical emails.
- Exchange server migration — you want to keep a portable backup before the migration.
- Switching to a non-Microsoft email client — convert the OST to PST, then to EML or MBOX for Thunderbird or Apple Mail.
- Forensic recovery — an OST on a recovered hard drive needs to be examined as evidence.
How to Convert OST to PST
- Go to mailtopst.com/convert/ost-to-pst.
- Upload the OST file.
- Download the resulting PST file and open it in Outlook — or convert further to EML/MBOX.
MailtoPst reads the OST binary structure directly, regardless of whether the Exchange account still exists. Files are encrypted via TLS, processed on EU servers, and auto-deleted. Fully GDPR compliant. Try free — no credit card required.
Frequently Asked Questions
Can I rename an OST file to PST?
No. Despite similarities in the binary architecture, the internal flags, encryption, and metadata structures differ. Simply changing the extension does not work — Outlook will refuse the file.
Which file is larger, PST or OST?
For the same mailbox content, OST files are typically slightly larger because they include synchronization metadata and index structures that PST files do not need. An OST for a 10 GB mailbox might be 12-15 GB.
Can I have both PST and OST files in Outlook?
Yes. Outlook commonly works with both simultaneously. The OST handles your live Exchange/M365 mailbox, while one or more PST files serve as local archives. They appear as separate folder trees in the Outlook sidebar.
What happens to the OST if I lose internet?
That is exactly what the OST is for. You can continue reading, composing, and organizing emails. When connectivity returns, Outlook syncs your changes with the server.
Should I back up my OST file?
Backing up the OST is generally unnecessary because it is a cache — the authoritative copy is on the server. However, if the server account might be deleted (e.g., leaving a job), converting the OST to PST before losing access is strongly recommended.
Upload your file now and convert your OST to a portable PST archive.