What is an OST File and What does It Store?
OST is short for Offline Storage Table. Outlook creates the file when you add an Exchange, Microsoft 365, IMAP or Outlook.com account.
The file holds a synchronized copy of your mailbox, so you can work with mail, contacts and calendars without a connection. Microsoft states that you do not need to backup OST data, because the server stores the live data.
| Detail | OST |
|---|---|
| Extension | .ost |
| Full form | Offline Storage Table |
| Used by | Exchange, Microsoft 365, IMAP and Outlook.com accounts |
| Contents | Synchronized copy of the server mailbox |
| File signature (hex) | 21 42 44 4E |
Where does Outlook Store the OST Data?
Outlook saves OST files in the AppData folder of your Windows profile. The path is drive:\Users\username\AppData\Local\Microsoft\Outlook.
- Open Outlook and select File.
- Select Account Settings and then Account Settings again.
- Open the Data Files tab.
- Select your account and then Open File Location.
Heads Up. AppData is a hidden folder. Show hidden items in File Explorer when you browse to it by hand.
How is OST Different From PST?
| Point | OST | PST |
|---|---|---|
| Role | Copy of a server mailbox | Local archive or POP mail store |
| Backup needed | No, because the server stores the data | Yes, because the PST is the only copy |
| Default folder | AppData\Local\Microsoft\Outlook | Documents\Outlook Files |
| Works on another computer | No, it links to one profile | Yes, you can copy it |
| Rebuild | Outlook downloads it again | You must restore a backup |
Our PST file extension guide explains the other format in detail.
How Large can OST Data Grow?
The default size limit is 50 GB, and administrators can adjust it. Microsoft adds that OST data grows 50 percent to 80 percent larger than the mailbox size that Exchange reports.
Outlook 2016 and later cache only the last 12 months of mail by default. Move the Mail to keep offline slider when you need older items on your computer.
Microsoft documents these limits in its Cached Exchange Mode article.
How do You Open an OST File?
Method #1: Open It Through Outlook (Free)
Who it suits. Users whose account is still active. Extra application. None.
Start Outlook with the profile that owns the file. Outlook opens the OST file in the background and shows your mail in the folder list.
Method #2: Open It Without the Account (Paid After Demo)
Who it suits. Users whose server account is deleted or unreachable. Extra application. An OST viewer or extractor.
Outlook cannot open OST data that belongs to another profile. An OST tool opens the file directly and exports mail to PST. See the OST extractor tool guide and the open an OST file thread for options.
How do You Fix Damaged OST Data?
The server holds your data, so a rebuild is the safest fix. Close Outlook and rename the file to end in .old. Start Outlook and let it download the mailbox again.
Microsoft says you can create a new copy at any time. Large mailboxes need hours to sync, so start the rebuild early in the day.
Three threads explain common messages. See errors detected in the OST file, OST error code 0000000E and cannot open OST file.
Frequently Asked Questions
What does OST Stand for?
OST stands for Offline Storage Table. Microsoft calls the file an offline Outlook Data File.
Where is OST Data in Windows?
The default folder is AppData\Local\Microsoft\Outlook inside your user profile. Open Account Settings and the Data Files tab to see the exact path.
Is a Backup of OST Data Required?
No. Microsoft states that OST data is a copy of server data, and Outlook can create a new copy at any time.
Can You Move an OST File to Another Computer?
No. The file links to one Outlook profile, so Outlook downloads new data on the new computer. Export to PST when you need a portable copy.
Is OST the Same as PST?
No. OST mirrors a server mailbox. PST stores archives and POP mail, so you must backup PST data yourself.
How Big can OST Data Get?
The default limit is 50 GB, and administrators can adjust it. The file grows larger than the server mailbox.
Can You Convert OST Data to PST?
Yes, with an OST converter. Use it when the server account is gone and the file is your only copy.
Author's Verdict
OST data is a safe copy of server data, so rebuild before you worry. The server stores your mail.
Export to PST only when the account is gone. That is the one case where the file is your only copy.
Key Takeaways
- OST stores an offline copy of a server mailbox.
- OST data needs no backup.
- A rebuild fixes most damaged OST files.