How to Convert OST to PST Online: 3 Methods

To convert OST to PST online, Stellar’s online OST to PST converter turns OST files under 500 MB into PST at no cost in the browser. It needs no Outlook installation or Exchange Server connection. OST files larger than 500 MB and up to 5 GB cost $39.99 per mailbox, so the free plan suits smaller mailboxes and Premium covers the rest of that range. We ran a sample OST through the free plan and checked the PST it produced against the original: every email and attachment came through.

An OST file is Outlook’s offline copy of a server mailbox, while a PST is a portable file you can open, import or back up. You can’t move an OST file from one computer to another, so a PST is how that mail travels. If the mailbox still works in Outlook, Outlook’s own export is another option, covered in Method 2.

Key Takeaways

• Stellar’s online converter is free for OST files under 500 MB, one file per session, and the PST stays available to download for 24 hours.
• Premium converts OST files up to 5 GB for $39.99 per mailbox and keeps the PST for 48 hours.
• In our test, a 3.89 MB OST converted in about 2.5 minutes and all 72 emails arrived in the PST.
• When the mailbox still works, remember that Outlook downloads only the past 1 year of email by default, so move the sync slider to All before you export.
• In Cached Exchange Mode, an export holds only the items in the current cache, which covers the past 12 months by default.
• In the Stellar Converter for OST desktop app, the Corporate Edition costs $79 and converts up to 5 OST files with no size limit.

Which OST to PST method fits your situation?

The table maps 5 common situations to a method, its cost, its size limit and whether Outlook must be installed. The deciding question is whether the mailbox still exists on Microsoft 365, Exchange or Outlook.com, so find your row, then jump to that method.

Source: Stellar Online OST Converter pricing, October 2026; Microsoft Support

If the account still signs in, the two Outlook rows cost nothing. For an orphaned OST, start with the free online plan.

Method 1: Convert OST to PST Online

Stellar’s Online OST to PST Converter needs no Outlook installation or Exchange Server connection and converts OST files directly from your browser, which suits an orphaned file. The tool supports OST files created by MS Outlook 2003, 2007, 2010, 2013, 2016, 2019, and 2021, including both ANSI and Unicode formats.

Steps to Convert OST to PST Online

1. Open the converter page and select Sign In.
2. Upload your .ost file in the first step of the wizard, which runs Upload OST, Conversion to PST, then Preview & Save.
3. Select Start Conversion. The page shows Analyzing, then Converting OST with a percentage, then Generating Preview.
4. When File Converted Successfully appears, select Preview to check the folder tree and item counts.
5. Select Save PST File, then select Download PST under the Free column of the pop-up that opens.
6. Import the PST into Outlook with the steps further down, or keep it as a backup.

Stellar says most OST files are converted and ready to download within 15 minutes, and larger OST files may take additional time.

What Happened When We Tested It

We used a public sample OST from Aspose’s email code examples on GitHub instead of a real work mailbox, so no private mail left our hands. We ran it in Brave on a Mac. Stellar read it as a 3.89 MB file, and the run from Start Conversion to the Save button took about 2.5 minutes. Most of that time went to the Analyzing and Generating Preview stages.

Two buttons need a quick note:

• The Preview button downloads an HTML file of the folder tree, so look in your Downloads folder.
• Save PST File opens a plan window, and the free Download PST button is in its Free column.

We opened the PST with readpst, an open-source PST reader, and compared it with the source OST. The counts matched: 12 emails in the Inbox, 25 and 29 in its two subfolders, and 6 in Sent Items. That makes 72 emails in all, with all 93 attachment parts present.

Free, Premium and Desktop Limits

Price is one of four things that separate the plans. Premium covers files up to 5 GB at $39.99 per mailbox. The table below adds the file count and download window, which also decide the plan.

Source: Stellar Online OST Converter pricing page, captured October 21, 2026

OST files are uploaded using SSL encryption and converted in a fully automated environment with no human touchpoints, and files are permanently deleted within 24 hours on the Free plan and 48 hours on Premium. For a work mailbox, check your company’s data policy before uploading.

For OST files up to 500 MB, you can preview the converted folder structure and item count in your browser before downloading. That preview is how we checked our counts before saving.

How can I convert an OST file to a PST file without using Outlook?

Use an online OST to PST converter, which doesn’t depend on Outlook. Stellar’s online tool requires no Outlook installation and no Exchange Server connection, so it works on a PC where Outlook was removed or the server is offline.

Is there a free OST to PST converter?

Yes, Stellar’s online converter is free for OST files under 500 MB. The Free plan allows you to convert only 1 OST file per session, and the converted PST file remains available to download for 24 hours. When the mailbox still exists, Outlook’s own export in Method 2 is free.

Before that export, it helps to know why an OST can’t simply be renamed.

OST vs PST: Why You Can’t Just Rename the File

Outlook Data Files (.pst) contain Outlook user messages and other Outlook items, such as contacts, appointments, tasks, notes, and journal entries, according to Microsoft. An OST mirrors a mailbox that lives on a server, so its format and purpose differ from a PST, and renaming the extension does not turn one into the other.

For Microsoft 365 accounts, Exchange accounts, or Outlook.com, Hotmail.com, or Live.com accounts not accessed by POP or IMAP, you won’t have a Personal Folders (.pst) file. Those accounts may have an Offline Folders (.ost) file, which Outlook automatically recreates when you add a new email account.

Some of the folders that hold these files might be hidden, and Windows might have file name extensions such as .pst and .ost turned off, so switch both on in File Explorer options before you hunt for the file.

That recreation offers a shortcut. If you’re using an Exchange email account, you can delete the offline Outlook Data File (.ost) and Outlook will recreate it the next time you open Outlook. So when the mailbox still exists, a stale OST is disposable: rebuild it, let it sync, and export a fresh PST.

What is the difference between an Outlook OST and PST file?

A PST is a standalone Outlook data file you can open, import and back up on any Windows PC with Outlook. An OST is the synchronized offline copy of a server mailbox, and you can’t move that file from one computer to another. Converting turns that cache into a portable PST.

When the mailbox still exists, that rebuild-and-export path is Method 2.

Method 2: Export to PST From Outlook When the Mailbox Still Works

Classic Outlook exports a mailbox through File, then Open & Export > Import/Export, and the wizard writes a PST with no extra software. This method needs the account to still sign in and sync.

Before You Export: Pull the Whole Mailbox First

By default, Outlook is set to download email for the past 1 year. If you want to export all your emails, select Account settings > Account Name and Sync Settings and move the slider to All. If you’re using Cached Exchange Mode, Outlook will only export the items in your current cache, which by default contains items from the past 12 months, so turn off Cached Exchange Mode first to export more.

Don’t export the .pst file to a OneDrive location, because OneDrive will block automatic syncing for Outlook .pst files.

Steps to Export the PST in Classic Outlook

1. At the top of the ribbon in classic Outlook, choose File, then Open & Export > Import/Export.
2. Choose Export to a file > Next.
3. Click Outlook Data File (.pst) > Next.
4. Select the name of the email account to export and make sure the Include subfolders check box is selected.
5. Click Browse to select where to save the Outlook Data File (.pst), enter a file name, and choose OK.
6. Choose Finish, and set an optional password if Outlook asks for one.

Only information for one account can be exported at a time, so repeat the wizard for each account. When Outlook exports emails, it includes any attachments to the emails, but it does not export folder properties such as views, permissions, and AutoArchive settings, message rules, and blocked senders lists.

How do I export my entire mailbox to a PST file in Office 365?

Add the Microsoft 365 account to classic Outlook, move the sync slider to All, turn off Cached Exchange Mode, then run the export above. The Include subfolders check box ensures that everything in the account will be exported: Inbox, Calendar, Contacts, and Tasks.

Export From New Outlook for Windows

In new Outlook for Windows, go to Settings > Files > Export and select Get started, then follow the steps in the wizard to select the mailbox or specific folders you want to export. Microsoft’s latest update added the ability to export emails, calendars, contacts, and tasks from a mailbox to a .pst file.

Reading that PST back inside new Outlook is narrower. Calendar and contacts items saved in .pst files are not available in new Outlook at this time, and opening a PST there carries four requirements:

• Classic Outlook must also be installed, and both versions need the same architecture, 32-bit or 64-bit.
• The account you’re using in new Outlook must have a Microsoft 365 subscription.
• Outlook Data Files (.pst) aren’t supported with the ARM version of new Outlook.
• New Outlook cannot be in Safe or Recovery modes.

Because every PST feature in new Outlook still leans on classic Outlook, the classic wizard stays the dependable route on any PC that has it.

Past 5 GB, or with several orphaned OST files to rescue, the online tool gives way to desktop software.

Method 3: Use Desktop Software for Large or Inaccessible OST Files

For the Stellar Converter for OST desktop app, the Corporate Edition costs $79 and converts up to 5 OST files with no size limit, and the PST stays on your PC.

When the Outlook profile is still configured, the desktop app finds the file for you: it auto locates and lists the OST files of the IMAP, Exchange or Microsoft 365 profiles configured in Outlook. It shows a detailed preview of the mailbox items before you save, and a single email can be saved as PST, MSG, EML, RTF, HTML or PDF.

Stellar says the desktop app converts inaccessible OST files to PST.

How to Import the Converted PST Into Outlook

Importing the PST uses the same Import/Export wizard that Method 2 used for the export, and the steps below work for a PST from any of the 3 methods, whether the online tool or desktop software produced it.

1. At the top of the ribbon in classic Outlook, choose File, then select Open & Export > Import/Export to start the Import/Export wizard.
2. Select Import from another program or file, and then select Next.
3. Choose the file type Outlook Data File (.pst) and select Next.
4. Browse to the .pst file you want to import, and under Options choose how you want to deal with emails and contacts.
5. If a password was assigned to the Outlook Data File (.pst), enter the password, and then select OK.
6. Choose the destination mailbox, or Import items into the current folder, then choose Finish.

When the progress box disappears, the import is finished.

New Outlook is less settled. Microsoft’s import article lists Settings, Data files, and then Import, with Get started under Outlook Data File (.pst), yet its export article still says importing a mailbox or calendar using a .pst file is currently not supported in new Outlook. If your build doesn’t show the Import option, turn off the New Outlook toggle and switch back to classic Outlook and use the steps above.

OST to PST Troubleshooting

Older mail can go missing because in Cached Exchange Mode the cache contains items from the past 12 months by default. An orphaned OST can’t hold mail it never synced, so messages older than the synced window aren’t in the file to convert.

• Old email missing: if the mailbox still exists, set the sync slider to All and turn off Cached Exchange Mode. Let Outlook resync, then export again with Method 2.
• Can’t find the free download: select Save PST File, then Download PST in the Free column of the plan window.
• PST won’t open in new Outlook: check that classic Outlook is installed at the same architecture and that new Outlook isn’t the ARM version.
• Export target is in OneDrive: move the target folder out of OneDrive, which blocks automatic syncing for Outlook .pst files.

Is the free online converter enough?

To convert OST to PST online, Stellar’s converter handles OST files under 500 MB at no cost, and files up to 5 GB cost $39.99 per mailbox. Download the PST as soon as the conversion finishes, check its folder counts, and check your data policy before uploading a work mailbox.

When the mailbox still signs in, Outlook’s own export in Method 2 is another route. Outlook downloads only the past 1 year of email by default, so lift that limit first. Larger, inaccessible or multiple OST files belong with desktop software. Microsoft plans to add bulk importing of Mail, Calendar, and Contacts from a PST into a mailbox in new Outlook and to remove the requirement that classic Outlook be installed, so check the new Outlook steps again once that update lands.