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OST vs PST: What Is the Difference?

By the Sensory Networks Editorial Team Updated July 3, 2026 6-minute read

Quick answer: PST is Outlook's portable archive file; OST is a synchronized offline copy of an Exchange or Microsoft 365 mailbox. An OST without its original account is unreadable in Outlook, so orphaned OST data is usually rescued by converting it. CoolUtils Total Outlook Converter reads both formats and exports mail to PDF, DOC, HTML, EML or a fresh PST in one batch, offline.

Both files sit on your disk, both end up holding gigabytes of mail, and both start with a letter and end in "ST". That is where the similarity stops — and mixing them up is how IT departments end up with archives nobody can open.

What is a PST file?

A PST (Personal Storage Table) is Outlook's self-contained mail store. Outlook creates one for POP accounts, for local archives, and whenever you run an export; it holds messages, contacts, calendar entries and attachments in a single file. The format is portable on purpose: copy the PST to another machine, attach it to any Outlook profile, and the mail opens. That portability is also why PSTs multiply — they get burned to backup drives, mailed between admins, and forgotten in shared folders. Microsoft documents the format family in its introduction to Outlook data files.

What is an OST file?

An OST (Offline Storage Table) is not an archive — it is a cache. When Outlook connects to an Exchange server, Microsoft 365 or an IMAP account, it keeps a synchronized local copy of the mailbox in an OST so you can read and write mail offline; changes sync back when the connection returns. The server copy is the master. Delete the OST and Outlook simply rebuilds it at the next sync — as long as the account still exists. That last clause is the whole problem.

PST and OST side by side

Property	PST	OST
Role	Standalone archive / local store	Offline cache of a server mailbox
Created by	POP accounts, archiving, exports	Exchange, Microsoft 365, IMAP sync
Portable	Yes — opens in any profile	No — bound to its account and profile
Master copy of the data	The file itself	The server (until the account dies)
If the file is deleted	Mail is gone unless backed up	Rebuilt on next sync — if the account exists
Typical trouble	Forgotten archives, oversized files	Orphaned after account or server removal

Why can't Outlook just open an OST file?

Because an OST is welded to the mail profile and account that created it. Attach a stray OST to a different profile and Outlook refuses; delete the account and the local cache is stranded. This is by design — the OST was never meant to live without its server. It becomes a real-world problem in three standard ways: an employee leaves and their account is deprovisioned before anyone exports the mailbox; an on-premises Exchange server is decommissioned; or a cloud tenant is closed during a merger. In each case the OST on the old workstation is suddenly the only copy of years of correspondence — readable by nothing in Microsoft's toolbox.

When do you actually need to convert?

Orphaned OST. The account is gone; the cache must be converted to PST or PDF to be readable at all.
Departing employees. HR or legal wants the mailbox preserved as PDF — the workflow in our eDiscovery export guide.
Archives for non-Outlook readers. Auditors, counsel and successors read PDF, not PST; see the step-by-step PST-to-PDF guide.
Client migration. Moving to Thunderbird or a web client means exporting mail as EML or HTML.

How to convert an OST or PST to something readable

Desktop converters read the file directly, so no server, password or original account is needed. With Total Outlook Converter you open the OST or PST, tick the folders, and export to PDF, DOC, HTML, TIFF, EML — or write everything into a fresh PST that any Outlook profile can attach. The batch runs offline, keeps the folder tree, and names files by date, sender or subject. You can download the free trial and test it on your own file for 30 days; no credit card or email is asked for. How it compares with the alternatives is in our 2026 Outlook converter ranking.

Frequently asked questions

Can I open an OST file on a computer without the original account?

Not with Outlook — it rejects any OST that does not belong to the current profile. A converter that parses the file directly, such as Total Outlook Converter, reads the messages regardless and exports them to PDF, EML, HTML or a new PST.

Can an OST file be converted back to PST?

Yes. Total Outlook Converter writes the OST contents into a fresh PST, folder structure included. The resulting file attaches to any Outlook profile like a normal archive, which is the cleanest way to bring a stranded mailbox back into circulation.

Why does Outlook say my OST cannot be opened?

Usually the account it belonged to was removed or the profile changed, so the binding check fails. Outlook has no import function for foreign OST files; the practical options are recreating the sync (if the account still exists) or converting the file with a third-party tool.

Where does Outlook keep PST and OST files?

Under your Windows user profile by default. The reliable way to locate them is inside Outlook: File, then Account Settings, then the Data Files tab lists every attached file with its full path.

Do attachments survive the conversion?

Yes. Total Outlook Converter saves attachments in their original format next to the exported messages, and the Pro edition can convert them and embed them inside the output PDF so each message becomes one complete file.