Free OST Converter
Convert to Any Format

OST (Offline Storage Table) is Microsoft Outlook's format for storing a local copy of your mailbox when you use Microsoft Exchange or certain other account types. The file allows Outlook to work offline and sync when connected. Like PST, the file begins with the magic bytes "!BDN" (0x21 0x42 0x44 0x4E). Byte 10 indicates the format: 14 for ANSI (older, 2 GB limit) and 23 for Unicode (larger files, international support). Converting OST means taking that binary offline cache and turning its contents into other formats — such as one EML file per message, a single MBOX mailbox, a PST-style container, CSV for spreadsheets, or HTML, TXT, or JSON for backup and analysis.

Our free OST Converter processes everything on your device. You drop a .ost file, and the tool validates the header (magic bytes, version, encryption), then scans the file for email-like content: readable header blocks (From:, To:, Subject:, Date:) and UTF-16LE text. Extracted messages can be exported as EML, MBOX, a PST-style structured export, CSV, HTML, TXT, or JSON. All processing happens on your device; nothing is uploaded. OST is a complex proprietary format, so this tool extracts as much as possible using advanced analysis. For full fidelity and a native PST that Outlook opens directly, Outlook or a dedicated desktop OST-to-PST tool may be needed.

Common conversion scenarios include recovering data when an Exchange account is no longer accessible, migrating from Outlook to Thunderbird (OST to MBOX or EML), backing up mail as EML, and moving to Gmail or Office 365 (export to EML/CSV first). Because the converter works online, you can use it on any operating system without installing Outlook. OST files are typically found on Windows where Outlook has been used with Exchange or an account that uses offline caching; converting them lets you free the data from that environment.