OST Viewer
Read Outlook OST offline mailbox files without Exchange or the original account. Browse, search and save emails.
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Runs entirely in your browser — nothing is uploaded
How to use the OST Viewer
- 1
Open your OST file
Click the box or drag the file onto it. It’s read by your browser — nothing is uploaded.
- 2
Read it instantly
The content appears on the page in a second or two, even for large files.
- 3
Convert if you need to
Use the related converters below to turn the file into PDF, CSV or another format.
An OST is tied to the Outlook profile that created it — open it here to get at the mail of an old or disconnected account. Folders, messages and attachments are shown right in your browser.
Read the full guide with screenshotsFrequently asked questions
Is the OST Viewer free?
Yes — completely free with no daily limit and no account. ZapMorph’s viewers and developer tools are unmetered.
Is my file uploaded to a server?
No. The file is opened by your browser and displayed on this page. It is never uploaded, stored or logged — close the tab and it’s gone.
Does it work on my phone?
Yes. It works in any modern browser on iPhone, Android, Windows, Mac and Linux, with nothing to install.
How large can the file be?
There is no fixed limit — it depends on your device’s memory. Files of a few hundred megabytes open fine on most computers.
Related converters
OST to PDF
OSTPDF
Convert Outlook OST files to PDF with sender, recipients, date and message body. Private — nothing is uploaded.
OST to EML
OSTEML
Convert Outlook OST files to standard EML files that open in Thunderbird, Apple Mail and Outlook.
OST to MBOX
OSTMBOX
Combine Outlook OST files into one MBOX mailbox for import into Thunderbird, Apple Mail or other clients.
OST to CSV
OSTCSV
Export Outlook OST files to a CSV spreadsheet — one row per email with date, sender, subject and body.
More file viewers
PDF Viewer
Open and read PDF files online with zoom and page navigation. Nothing is uploaded — the PDF opens on your device.
Word Document Viewer
View DOCX files online without Microsoft Word. Headings, lists, tables and images are shown. Private.
EML Viewer
Open .eml email files online and read the message, headers and attachments. Nothing is uploaded.
MSG Viewer
Open Outlook .msg files on Mac, Windows, Linux or phone — read the email and save attachments.
PST Viewer
Open Outlook PST files without Outlook: browse folders, search and read emails, and save attachments. Private.
Excel Viewer
Open XLSX spreadsheets online and browse every sheet as a table — no Excel required. Private, in-browser.