PST vs OST Files in Outlook: What Is the Difference?

A PST file and an OST file are both Outlook data files, they share most of the same on-disk structure, and they show the same messages when everything is working. Yet only one of them survives a machine dying with your account still attached. The other becomes a 12 GB paperweight the moment the profile that owned it goes away.

This guide explains the PST vs OST file difference in the terms that matter when something has gone wrong — what each file is, what it is bound to, where Outlook keeps it, what happens when you remove the account, and what a person can genuinely recover from a leftover file after a computer dies. It also covers the one honest thing to do with an orphaned OST, and why the popular "just convert it to PST" advice is usually pointing at the wrong tool.

The verdict, up front#

A PST (Personal Storage Table) is a portable local Outlook data file. You own it, it lives wherever you put it on disk, it is self-contained, and any Outlook profile on any machine can attach it and read every message inside. That is what makes a PST a real archive: the mail is in the file, not somewhere the file is pointing at.

An OST (Offline Storage Table) is a synchronised cache. It mirrors the contents of a specific server mailbox — Exchange, Microsoft 365, Outlook.com or IMAP — while Outlook is offline, and it is bound to the exact profile plus mailbox identity that created it. Delete the profile or lose access to the account, and the OST is orphaned. Outlook will not attach an orphaned OST; the format was not designed to be opened stand-alone.

So the short answer to "what is the difference between a PST file and an OST file in Outlook" is: a PST is storage you own, an OST is a copy of storage you rent. The rest of this page turns that difference into the practical decisions — which one is a backup, which one you can move to a new computer, and what to do when all you have left is the OST.

At a glance: PST vs OST#

The dimensions below are the ones that decide the outcome. Draft quality, search behaviour, and how each file feels day-to-day are almost identical while both are working; the structural differences only show up when you try to move a file, lose an account, or read one on a different machine.

Where the PST file wins#

The PST is the file to reach for when you want something you can hold. It is a single self-contained archive with a well-documented format (Microsoft publishes the MS-PST specification), and every version of classic Outlook can open one another version wrote. If the machine goes away, the PST does not go with it — the mail is in the bytes.

That makes the PST the correct file for four jobs. Long-term archives that need to outlive whatever mail account you happen to have this year. Exports before a migration, so you have a fallback if the migration goes badly. Local storage for a POP3 account, which is the one account type that has no server copy to fall back to. And handing a set of messages to someone else — a lawyer, a compliance team, a new employer — as a single portable file rather than a folder of exported EMLs.

The reason people still export to PST rather than a more open format is that Outlook is the tool that reads it best. Third-party clients can be pointed at a PST with mixed results, but Outlook opens one instantly and preserves categories, flags, meeting responses and RTF-formatted bodies that other formats lose. If you have Outlook and you want a single-file archive that will still open in ten years, the PST is the format that gets you closest to that.

The one honest caveat: a PST password is not real security. Microsoft has said as much in its own documentation, and there are free utilities that remove one in seconds. If a PST is going to leave your machine, put it inside an encrypted container — an encrypted disk image, a 7-Zip archive with a strong password, or BitLocker on the drive that carries it — and treat the PST's own password as a speed bump, not a lock.

Where the OST file wins#

The OST is not a competing archive format; it is a piece of infrastructure that makes an online mailbox feel local. Its two real advantages come from that role.

First, offline-with-catch-up. When you disconnect from the network, the OST is what Outlook is reading — every message that has been synced is available immediately, and every action you take is queued. When the connection returns, Outlook reconciles the queue with the server: drafts land, deletes propagate, moves apply. You never manage that process; the OST is where it happens. A PST cannot do this because it has no server behind it to reconcile with.

Second, startup performance. Loading a full mailbox from an Exchange server at every launch would be slow and would hammer both sides. The OST lets Outlook draw the interface off a local file and let the sync run in the background, so a mailbox with fifteen years of mail opens in seconds instead of minutes. On slow connections and on planes, this is the whole reason cached mode exists.

The trade is that the OST is not yours in the way a PST is. It is derived data — a projection of the server mailbox at the moment it last synced — and Outlook treats it that way. If the file is corrupted, the fix is usually to delete it and let Outlook rebuild it from the server; if the profile is deleted, the OST is orphaned and there is no supported way to open it back up. The OST is doing something a PST cannot, and paying for it with portability.

Where each file lives on disk#

The default locations for classic Outlook on Windows are worth knowing before you go looking for them. Both are technically hidden — Outlook does not surface the path in its main UI — and the difference in location is a hint about how each file is treated.

PST files, by default, land in Documents\Outlook Files under your user profile. That folder is user-visible on purpose, because the PST is a document you own — the same as a Word file. It is included in normal file backups and can be moved between drives with Outlook closed.

OST files land in the hidden folder %LOCALAPPDATA%\Microsoft\Outlook, which on most machines resolves to C:\Users\<you>\AppData\Local\Microsoft\Outlook. That path is deliberately buried, because Outlook treats the OST as application data rather than a document — something that can be regenerated from the server if it goes wrong. You can find either path from inside Outlook via File > Account Settings > Data Files, which shows every attached file and its location.

The macOS story is different in structure but the same in principle. Classic Outlook for Mac keeps its data in a package under ~/Library/Group Containers/UBF8T346G9.Office, and the new Outlook — on both Windows and Mac — has moved away from the .ost file entirely in favour of its own local cache format. If you are on new Outlook, you do not have an OST to worry about; you also do not have a portable PST-shaped file to reach for.

The cost of trying to recover an orphaned OST#

The reason people search for the PST vs OST difference is usually not curiosity. It is that a machine has died, the account it hosted is gone, and there is an OST file on a recovered drive. The natural question is whether that OST can be turned into a PST — and the honest answer is complicated.

There is no Microsoft-supplied tool that reads an orphaned OST. Outlook will not attach one, and the built-in scanpst.exe repairs a PST but does not convert an OST. Third-party OST-to-PST converters exist and some of them work, but the category is messy: pricing shape is usually a per-seat licence with a free preview that shows only some folders, quality varies wildly, and several products are wrappers around the same underlying engine. Do not print a price for one of these tools — check the vendor's own site at the time of purchase, and always run the free preview against your actual file before paying, because a converter that cannot open your OST will not be able to convert it either.

The much better answer, when the account still exists somewhere, is to bypass the OST entirely. Add the same account to a fresh Outlook profile on any machine, let it re-sync, and export the mailbox to a fresh PST from that profile. The server has the mail; the OST was only ever a copy. If the account is truly gone — the mailbox was deleted, the tenant is closed, the IMAP provider shut down — a converter is your only path, and you should treat any recovered messages as a bonus rather than the default outcome.

Who each file is genuinely for#

Provider setup and what a person needs to do next decide this more than format preference. The table below matches the two files to real situations.

A third option, honestly#

We build AI Emaily, so this section is the disclosure and the recommendation in the same place. AI Emaily is a modern AI-native email client that connects Gmail, Outlook and standard IMAP in one inbox — it is not an Outlook data-file tool, and we do not open PSTs or OSTs. If you landed here to rescue an orphaned OST from a dead machine, the OST converter route above is the honest answer, not us.

Where AI Emaily fits into the PST vs OST question at all is on the account you still have. When the mailbox is live on Exchange, Microsoft 365, Gmail or IMAP, AI Emaily connects to it directly via OAuth or IMAP, syncs its own cache, and gives you a triage-focused interface with an AI agent that can act — with a Copilot mode that requires approval before every send, a full undo, and an audit log you can read. The OST question does not come up because we do not use OST files; we maintain our own local cache and the server mailbox is always the source of truth.

Our packaging is a 7-day free trial on Pro and Autopilot plans — card required, $0 if you cancel before day 7 — rather than a permanent free tier. Check pricing at aiemaily.com/pricing for the current tiers. On desktop we ship real downloadable apps for macOS (Apple Silicon) and Windows, and a native iOS app; Android is a PWA and there is no native Linux build.

Where AI Emaily is the wrong answer for this specific question: if the point of the exercise is opening a leftover .pst on a lawyer's machine, use classic Outlook — that is the tool the format was designed for. If the point is turning an orphaned .ost back into readable mail, use one of the converter utilities and preview before you pay. AI Emaily is what to reach for after those files exist, when you want the current account run properly rather than a historical archive re-opened.