OST

OST file format
explained

Offline Storage Table — Outlook's local cache of Exchange mailboxes

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What is a OST file?

OST (Offline Storage Table) is Microsoft Outlook's offline cache format for Exchange, Microsoft 365, and IMAP mailboxes. Unlike PST files that are standalone, OST files are synchronized copies of server-side data. When Exchange connectivity is lost, OST files let you continue working offline. OST files cannot be opened directly — they must be converted to another format first.

Origin

Microsoft Outlook + Exchange

Used by

Outlook connected to Exchange Server or Microsoft 365

Key features

Offline copy of server mailbox
Auto-syncs with Exchange/M365
Cannot be opened without the original account
Same internal structure as PST
Requires conversion for data recovery

OST conversions

Convert from or to OST with MailtoPst

Convert OST to...

OST→EML OST→Gmail OST→iCloud OST→IMAP OST→MBOX OST→MSG OST→Office 365 OST→PST

OST guides & articles

Convert OST to PST Free — 3 Methods That Actually Work (2026)

3 free methods to convert OST to PST in 2026. Online tool (no install), Outlook archive (if profile works), and desktop software. Includes orphaned OST recovery.

Convert OST to PST Free — Without Exchange or Outlook

Convert any orphaned OST file to PST format for free — no Exchange server, no Outlook needed. Online tool, EU servers, results in minutes.

How to Read an OST File Without Exchange Server (Free)

Open and read orphaned OST files without an Exchange server. Free online method — no Outlook, no desktop software, no server connection needed.

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OST file formatexplained