OST vs PST: What Is the Difference?
Quick answer: An OST is an offline cached copy of a mailbox that lives on the server (Exchange or Microsoft 365) and is tied to one Outlook profile. A PST is a standalone, portable Outlook data file you own and can move between PCs. You convert an OST to PST when the account is gone or the profile breaks and you still need the mail.
OST and PST are both Outlook data files, and both end in mailbox data — but they play opposite roles. One is a disposable local cache; the other is the copy you keep.
What is an OST file?
An .ost (Offline Storage Table) is a synced copy of a mailbox that Outlook keeps on your disk so you can read and write mail while offline. The master copy lives on the server — Exchange, Microsoft 365 or an IMAP host — and the OST re-syncs when you reconnect. It is encrypted and bound to the exact Outlook profile that created it, which is why you cannot just copy an OST to another computer and open it.
What is a PST file?
A .pst (Personal Storage Table) is a standalone Outlook data file. It is not tied to a server or a profile: you can copy it to a USB stick, attach it to an email, back it up, or open it on any PC with Outlook or a PST viewer. PST is the format people mean when they talk about archiving, exporting or handing over a mailbox.
OST vs PST at a glance
| OST | PST | |
|---|---|---|
| Role | Offline cache of a server mailbox | Standalone, portable archive |
| Tied to | One Exchange / 365 profile | Nothing — fully portable |
| Portable | No — will not open elsewhere | Yes — open on any PC |
| Created by | Outlook, automatically | You, via export or a converter |
| Survives account deletion | Becomes orphaned / unreadable | Yes |
| Good for | Working online/offline | Backup, migration, handover |
Why would you convert an OST to a PST?
Because the moment the server side changes, the OST stops being useful. Common triggers:
- An employee leaves and the Exchange account is disabled — the OST is now orphaned.
- You migrate to a new server or a new PC and need the old mail in a portable file.
- The Outlook profile breaks and only the local cache is left.
- A lawyer or auditor is handed a raw .ost as evidence and needs to open it.
In every case the fix is the same: turn the OST into a PST you can open anywhere. The CoolUtils OST to PST Converter does this without Exchange or Outlook, even from a corrupted file. For the step-by-step, see how to convert OST to PST; to pick a tool, read our comparison of the best OST to PST converters.