Sign up free

OST to HTML Converter

Convert Outlook OST files to HTML pages that open in any browser, with headers and formatting kept.

Your files never leave your device

How to convert OST to HTML

  1. 1

    Choose your OST file

    Click the upload area or drag files onto it. Nothing is uploaded — files are opened locally.

  2. 2

    Start the conversion

    Press the convert button. Your browser converts the files in seconds.

  3. 3

    Download the result

    Save each file individually or everything at once as a ZIP.

Read emails from Outlook’s offline Exchange and Microsoft 365 mailboxes without the original app. The HTML body is kept as sent, with a header block on top; plain-text emails are wrapped so they stay readable.

Read the full guide with screenshots

What is OST?

OST is Outlook’s offline copy of an Exchange or Microsoft 365 mailbox. It has the same structure as a PST but is normally tied to one account — converting it rescues mail from old or orphaned profiles.

What is HTML?

HTML is the markup language of web pages. Converting to or from HTML lets you publish content online or bring web content into other tools.

Is it private?

Yes. Unlike upload-based converters, ZapMorph processes files inside your browser tab, so the file stays on your device. It is never transmitted, stored or logged — close the tab and it’s gone.

Frequently asked questions

Is the ost to html converter free?

Yes. Convert 1 file a day without an account, or 5 files a day with a free account. Pro removes all daily limits.

Are my files uploaded to a server?

No. The conversion runs entirely in your browser, so your files never leave your device. We only count how many files you convert — never their names or contents.

What is the maximum file size?

Up to 25 MB per file without an account, 50 MB with a free account, and 1 GB on Pro. Very large files also depend on your device’s memory.

Can I convert multiple OST files at once?

OST to HTML Converter works on one file at a time and can produce many output files, which you can download together as a ZIP.

Does it work on Mac, Windows, iPhone and Android?

Yes. It works in any modern browser — Chrome, Edge, Firefox and Safari — on desktop and mobile, with nothing to install.

Related converters

EML to HTML

EMLHTML

Convert EML files to HTML pages that open in any browser, with headers and formatting kept.

MSG to HTML

MSGHTML

Convert Outlook MSG files to HTML pages that open in any browser, with headers and formatting kept.

MBOX to HTML

MBOXHTML

Convert MBOX mailboxes to HTML pages that open in any browser, with headers and formatting kept.

PST to HTML

PSTHTML

Convert Outlook PST files to HTML pages that open in any browser, with headers and formatting kept.

OST to PDF

OSTPDF

Convert Outlook OST files to PDF with sender, recipients, date and message body. Private — nothing is uploaded.

OST to Text

OSTText

Extract plain text — headers and body — from Outlook OST files. Ideal for search, archiving and AI tools.