Quick answer: PST is a portable personal archive that stores your Outlook emails, contacts, and calendars locally for backup or migration. OST is an offline cache bound to one Exchange or Microsoft 365 profile, so it cannot be imported directly. MSG is a single saved message. You convert OST or MSG to PST to move mail between accounts and computers.

Key Outlook Data Formats

PST
Personal Storage Table
.pst files

PST files are Microsoft Outlook's primary data storage format, designed to store emails, calendars, contacts, tasks, and other mailbox items locally on your computer.

Purpose & Usage
Store complete mailbox data including emails, calendar events, contacts, tasks, notes, and journal entries for offline access and backup.
File Types
ANSI PST: Legacy format with 2GB size limit, compatible with older Outlook versions.
Unicode PST: Modern format supporting up to 50GB, used in Outlook 2003 and later.
Import/Export Scenarios
Ideal for transferring data between Outlook installations, creating backups, archiving old emails, and migrating to new computers or email systems.
OST
Offline Storage Table
.ost files

OST files serve as local cache for Exchange Server or Office 365 mailboxes, enabling offline access to your email data when not connected to the server.

Purpose & Usage
Synchronized cache of Exchange/Office 365 data that allows offline work. Automatically syncs changes when connection is restored.
Key Differences from PST
OST files are tied to specific user accounts and cannot be directly imported into Outlook. They're encrypted and server-dependent.
Why do I need to convert OST to PST?
To access OST data independently or transfer it to another system, conversion to PST format is required since OST files cannot be directly opened without the original Exchange connection. A dedicated tool such as Total Outlook Converter exports OST and PST mail to PST, PDF, and other formats.
MSG
Outlook Message
.msg files

MSG files contain individual Outlook items (emails, calendar events, contacts) with all associated metadata, attachments, and formatting preserved in a single file.

Purpose & Usage
Store individual Outlook items as standalone files that can be opened, shared, or archived independently of Outlook's main data files.
Common Use Cases
Drag-and-drop emails from Outlook to desktop, share specific messages with others, create email templates, or archive important communications.
Content Preservation
Maintains complete message integrity including sender/recipient info, timestamps, attachments, formatting, and custom properties.
What Is the Difference Between PST, OST and MSG?
Format
PST
OST
MSG
File Size
Up to 50GB (Unicode)
Up to 50GB
Individual items
Portability
Highly portable
Account-specific
Fully portable
Direct Import
Yes
No
Yes

How Do I Know If My File Is PST, OST or MSG?

Follow these steps to determine what type of Outlook data file you are working with

1
Check File Extension
Look at the file extension in Windows Explorer or File Manager. This is the most reliable indicator of file type.
2
Examine File Size
MSG files are typically small (KB to MB), while PST/OST files can be very large (GB). This helps distinguish between individual items and data stores.
3
Check File Location
PST files are often in Documents folder, OST files in AppData/Local/Microsoft/Outlook, and MSG files wherever they were saved.
4
Try Opening in Outlook
PST and MSG files can be opened directly in Outlook. OST files cannot be opened if they're from a different user account or computer.