Free online OST to XLSX converter

What the OST format is

OST is an Outlook data file that stores a local, synchronized copy of mailbox content for Microsoft Exchange, Microsoft 365, and some IMAP accounts. Microsoft Outlook creates it when an account is configured for cached or offline access; the file can contain mail, contacts, calendar entries, tasks, and folder metadata.

An OST is tied to its Outlook profile and account. It is not a portable archive like PST, and Outlook normally cannot open an orphaned OST after the original profile or server account is unavailable.

What the XLSX format is

XLSX is the XML-based workbook format used by Microsoft Excel, LibreOffice Calc, and other spreadsheet applications. It supports worksheets, filters, formulas, tables, formatting, charts, and structured analysis.

XLSX is appropriate when OST content needs to become rows and columns: messages can be represented by sender, recipient, subject, date, and size; contacts by names, addresses, phone numbers, and email fields; and calendar items by subject, start time, end time, location, and attendees.

How to actually convert OST to XLSX

Use Outlook, then Excel

1. Open Microsoft Outlook with the profile that can access the OST data.
2. Select the mailbox folder to export. Outlook exports one selected folder at a time, so repeat the process for mail, contacts, calendar, or other folders as needed.
3. Choose File → Open & Export → Import/Export.
4. Select Export to a file → Comma Separated Values, then select the required folder and save the result as a .csv file.
5. Open Excel and choose Data → From Text/CSV. Set the correct delimiter, text encoding, date interpretation, and column types before loading the data.
6. Save the imported workbook with File → Save As and select Excel Workbook (*.xlsx).

For contacts, Outlook may provide a field-mapping screen; map custom fields before completing the export. For mail, export headers and body fields that Outlook exposes, then inspect the resulting columns in Excel.

Use the built-in online converter

You can convert the file on 101convert.com for free without installing software: upload the OST file, select XLSX as the output format, start the conversion, and download the resulting workbook. Use this method only when the OST contains data the converter can read and your organization permits uploading mailbox data to a web service.

When Outlook cannot open the OST

An OST disconnected from its original account generally requires a desktop OST recovery or extraction utility. Export the recovered mail, contacts, or calendar data to CSV or another spreadsheet-compatible format, then open it in Excel and save it as .xlsx. Do not delete the original OST until the exported records and attachments have been checked.

Quality, compatibility, and limitations

• OST-to-XLSX is a data extraction operation, not a format-preserving conversion. Email threads, Outlook flags, folder hierarchy, embedded images, rules, and attachment relationships may not survive as spreadsheet fields.
• Attachments are usually exported separately or represented only by filenames and paths; XLSX does not reproduce an Outlook mailbox.
• CSV can corrupt leading zeros, long numeric strings, and dates when Excel guesses column types. Import through Data → From Text/CSV and set sensitive columns to Text.
• Large mailboxes may exceed Excel's limit of 1,048,576 rows per worksheet. Split the export across worksheets or files when necessary.
• Check timezone, regional date settings, Unicode characters, duplicate records, and empty fields after conversion. Preserve the original OST as the source copy.