OST File Extension
Outlook Offline Folder Format
- File Type ost
- Developer Microsoft
- Popularity
- Application Microsoft Outlook
- Category Data Files
- Format Binary
What is a OST file?
Files with the OST extension are specifically tied to Outlook Offline Folder Format, a Data Files type, and there are 6196 files in this category.
Microsoft, the developer of the Outlook Offline Folder Format (OST), has released file format and 242 programs designed to handle them.
OST files are compatible with various operating systems, including Mac OS, Windows and Linux. There are 7 programs available to open a OST file, including popular tools like Microsoft Outlook, EML Viewer Pro and SysTools OST Recovery.
File Signature
HEX code: 21 42 44 4E. ASCII code: !BDNProgram.
Programs that support OST files
How to open a OST file?
If you are having trouble opening a OST file, it usually means the right software is not installed. Choose a program from the list that matches your operating system. Next, visit the official website of the developer, download the software, and install it on your device.
Microsoft Outlook, EML Viewer Pro and SysTools OST Recovery are among the most widely used programs for handling OST files. To set up the newest version of Microsoft Outlook on your device, simply go to the official Microsoft website. There you can download the edition compatible with every operating system.
After successful installation, your operating system should seamlessly handle OST files via Microsoft Outlook.
Set Microsoft Outlook as the default OST handler
Choose an app to open a file on Mac
- Right click or use Control + left click on the desired OST file to launch the file options menu;
- Choose "Open in Application" and click "Other";
- Go to the lower section of the window, where you will find the "Enable" menu. By default, it is set to "Recommended Programs";
- Select "All Programs" from this menu and proceed to search for Microsoft Outlook. Check the box next to "Always open in app" to set it as the default option.
Change default programs in Windows
- To get started, right-click your OST file, then choose "Open With" and select "Choose Another Application" from the context menu;
- In the pop-up window, find and select the Microsoft Outlook application from the available options;
- Make sure the "Always use this app" checkbox is checked and click "OK" to confirm your choice as the default app for opening OST files.
How to edit a OST file?
To modify OST files, it is necessary to install dedicated software that lets you open and edit data files. Online OST editors generally offer fewer tools compared to programs designed for Outlook Offline Folder Format.
Best OST editors 2026
With the mentioned criteria in mind, we have assembled a selection of OST editors.
| OST Editor Software | Developer | Platforms | Overall Rating |
|---|---|---|---|
| Microsoft Outlook | Microsoft | Windows, Mac OS | 4.8 out of 5 (43 reviews) |
Convert files to and from OST
If you struggle with a OST file, consider converting it to alternative formats such as PS, MSG or other similar file types.
It is important to understand that converting a OST file might limit certain functions, but this conversion is usually reversible.
Convert OST to other file formats
Effective solutions for OST file problems
OST files may present some challenges, but most can be easily addressed with minor software tweaks. See the troubleshooting tips below for guidance.
What are the steps to repair a damaged OST file?
Launch Advanced Exchange Recovery or Kernel and open the damaged OST file. Then, choose the "Repair" option from the menu. Wait for the recovery process to finish as it will automatically restore the damaged data files.
How can I determine if my OST file is damaged?
The signs of corrupted OST data files can manifest as:
- The file causes Microsoft Outlook or other software to hang or crash during various operations;
- Error notifications appear during file manipulation;
- The file encounters problems during the opening process or shows error indications;
- An unexplained increase in the size of the OST file;
- The file shows slow performance when edited in Microsoft Outlook;
What are the reasons behind Microsoft Outlook crash?
Insufficient software updates are a likely cause of Microsoft Outlook crashes. Be sure to install the latest updates for Microsoft products.
Restarting your computer can resolve numerous common problems, so it should be the initial step before changing any software-related settings.