OST File Documentation

Overview

Feature Value
File Extension .ost
Full Name Outlook Offline Folder File
Description Stores offline copies of Exchange or IMAP email accounts
Developer Microsoft
Primary Association Microsoft Outlook
MIME Type application/octet-stream
Encryption Can be encrypted with Microsoft's file encryption
File Structure B-tree structure for storage
Can Store Emails, contacts, tasks, calendar items, and other user data
Synchronization Can synchronize with an Exchange server when connection is available
File Location (Windows) C:Users\%username%AppDataLocalMicrosoftOutlook
Accessibility Accessible only by Microsoft Outlook
Recovery Possible to recover data from corrupted files with specialized tools
Compatibility Compatible with all versions of Microsoft Outlook from 97 to Outlook 2019
Size Limit By default, 50 GB on newer versions of Outlook, customizable through registry
Difference from .PST .OST is for offline access, while .PST is purely for storage
Editable Can be edited offline; changes sync when online
Backup Not intended for backup; designed for temporary offline access
Advantages Allows working with Outlook data without an active network connection
Disadvantages Can become large and unwieldy; potential synchronization issues