Convert OST to HTML Online

The free online mail converter that transforms EML, MSG, PST, and MBOX files into PDF, HTML, Word, and more — no software required.

CoolUtils online mail converter supports all major email formats as input — EML, MSG, PST, OST, and MBOX — and converts them to PDF, HTML, DOC, TXT, JPEG, and other output formats. Upload the file, select the output format, and download the converted document in seconds. No installation, no registration, completely free.

CONVERT OST to HTML ONLINE

1) Upload OST file to convert

Drop files here, or Click to select

0 %

Allowed file types: pst, ost, eml, msg, mime, smime, p7m, mbox, dbx, vcf, vmbx, opf, asice, cpgz, lzh, zcf

2) Set converting OST to HTML options

3) Get converted file

DOWNLOAD CONVERTED FILE

Full version

Get Total Outlook Converter to convert your Outlook emails safely.

Total Outlook Converter To Convert Outlook emails

Convert Outlook emails in batch!;
Convert PST to PDF, DOC, TIFF, TXT, HTML, EML and more
Convert hundreds of files in 3 clicks;
Have a desktop converter that works without Internet;
Keep your emails safe, don't send them to the web;
Get more accurate results with the more poweful engine.

See features

NOW 20% OFF - $75.00 $52.50

*Only this month. Not for resellers.

Total Outlook Converter

Convert Outlook emails from multiple sources to a variety of formats.

Filter emails by sender, subject, date, keywords, and more.

Useful data saving options - convert only necessary data from your emails.

Unique output files formatting options specify how your files will look.

Affordable Price - pay once for a lifetime license with no hidden fees.

Command Line Support - convert emails via command line.

Fast batch conversion - convert multiple emails simultaneously.

High security options - protect your output PDF files (if you choose PDF).

Various file naming and date saving options - organize your output files easily.

Variety of attachment saving options - choose how to save attachments.

Trust - you can rely on powerful email converters from CoolUtils.

Easy to use even for beginners - enjoy clear user-friendly interface.

How to convert OST to HTML?

💾 Upload Your File: Go to the site, click on «Upload File,» and select your OST file.

✍️ Set Conversion Options: Choose HTML as the output format and adjust any additional options if needed.

Convert and Download: Click 👉«Download Converted File»👈 to get your HTML file.

Cloud Based

There is no need to install any software to convert OST to HTML with CoolUtils. Simply access the internet, and you can easily convert your file online with our service.

Secure

Our hardware operates in top-tier, highly secure data centers equipped with advanced electronic surveillance and multi-factor access control systems.

Ease of Use

Converting OST offline Outlook cache to HTML online has never been so simple. Drop your .ost file and the converter writes browser-readable HTML — perfect for web archival and sharing emails without Outlook.

Privacy

We understand the importance of privacy, which is why we've developed a comprehensive privacy policy detailing how we handle your files and personal information. You can read the policy here.

OST File

File extension	.OST
Category	File
Description	OST is an offline data file used by Microsoft Outlook to store emails, calendars, and other information when working in cached mode with an Exchange server.
Associated programs	Microsoft Outlook, SysTools OST Viewer, Kernel OST Viewer
Developed by	Microsoft
MIME type	application/vnd.ms-outlook
Useful links	More detailed information on OST files
Conversion type	OST to HTML

HTML File

File extension	.HTML
Category	Document File
Description	HTML allows creating structured documents and web pages. HTML file contains command tags in angular brackets that mark out each element like title, paragraphs, quotes, links, lists, etc. Web-pages generated with HTML have .html extension (for a static web-page). HTML code is processed by a browser, while user sees only the text and other elements of the page without any tags. The source code can be viewed separately. HTML files can be edited in a common text editor like a TXT file.
Associated programs	Any Web Browser (e.g. Internet Explorer, Safari, Firefox, Google Chrome)
Developed by	World Wide Web Consortium & WHATWG
MIME type	text/html
Useful links	More detailed information on HTML files

2026-01-22

We needed to archive an OST mailbox as browsable HTML for an HR investigation — the output was readable in any browser and easy to share with the review committee.

Patricia Nkemdirim

2026-03-11

Converted an executive's OST to HTML files for a compliance review; the folder-based HTML archive was straightforward to navigate and index.

Sven Eriksson

2026-05-04

Used this to create a web-viewable copy of an orphaned Exchange OST for legal discovery; reviewers could browse the archive without needing Outlook installed.

Fatou Diagne

Rating OST to HTML

4.6 (290 votes)Rate It

OST to HTML Conversion FAQ ▼

Do I need the original Exchange profile or password?

No. The converter reads the OST file structure directly without needing the original Outlook profile or Exchange credentials. That is the main reason to convert orphaned OST files — they can no longer be opened by Outlook once the profile is gone.

Will folder structure from the mailbox be preserved?

Yes. The HTML output mirrors the OST folder hierarchy — Inbox, Sent Items, Drafts, Archive, plus any user-created folders — as subdirectories. Open the index page in a browser to navigate.

Will HTML formatting from the original email be preserved?

Yes. Modern Outlook stores HTML-formatted messages, and that markup is preserved in the output. Inline images, fonts, colors, and tables render in the browser exactly as they did in Outlook.

Are inline images embedded?

Inline images referenced by 'cid:' in the original HTML are saved to a parallel folder and the tags are rewritten to point at the saved files. Open any HTML file directly and the images appear inline.

What about attachments?

Non-image attachments are saved alongside the HTML in an attachments subfolder, with the HTML file containing download links. The desktop Total Outlook Converter offers richer control over attachment handling.

Does this handle encrypted or signed messages?

Plain mail and unencrypted signed messages convert correctly. S/MIME-encrypted messages cannot be decrypted without the recipient's private key — the OST stores them in encrypted form.

How large an OST can I upload?

Up to 50 MB. Real OSTs from an active mailbox routinely run multi-gigabyte — for those, use the desktop Total Outlook Converter, which has no file size cap.

Convert OST to HTML — Browse an Orphaned Outlook Cache in Any Browser

OST is the offline mailbox cache Outlook keeps for an Exchange or Microsoft 365 account — and it is locked to the original profile. HTML opens in any browser. Converting OST to HTML rescues messages from an orphaned cache and produces a clickable archive you can read in Chrome, Firefox, Safari, or any browser without Outlook, Exchange, or even Windows. Drop your .ost file above.

What Is OST?

OST (Offline Storage Table) is the local mailbox cache Outlook creates when configured with Cached Exchange Mode or a Microsoft 365 account. The .ost file mirrors the server-side mailbox so Outlook can work offline. Unlike PST, OST is permanently bound to the original Outlook profile and the original Exchange account — if the profile is removed or the user is gone, the OST is effectively orphaned and unreadable by Outlook.

What Is HTML?

HTML (HyperText Markup Language) is the W3C standard for web documents, the format that built the web. HTML files render in every browser on every operating system — Chrome, Firefox, Safari, Edge — without any plugins or special viewers. HTML is also fully searchable as plain text and indexable by search and archive tools.

OST vs HTML — Format Comparison

Property	OST	HTML
Type	Outlook offline cache (binary)	Web document (text)
Developer	Microsoft	W3C / WHATWG
Readable without Exchange	No (Outlook needs original profile)	Yes (every browser, every OS)
Profile binding	Locked to original Outlook profile	None — fully portable
Native readers	Outlook with original profile	Chrome, Firefox, Safari, Edge
Searchable	Within Outlook only	Browser Find + grep on disk
Typical use	Cached mailbox	Web archive, exhibits, portable mail

When Do You Need OST to HTML Conversion?

Read an orphaned OST file in any browser without rebuilding the original Outlook profile
Build a portable web archive of a departed employee's cached mailbox
Publish selected emails to an internal wiki, intranet, or evidence portal
Open an OST on macOS or Linux, where Outlook is not available
Hand a reviewer a clickable folder structure of emails in HTML for discovery review
Index cached mail with a search engine that consumes HTML rather than PST/OST

Command-Line Conversion

Batch-convert an entire folder without opening the interface:

TotalOutlookConverter.exe C:\ost_files C:\html_out /ConvertTo html /ProcessRecursively

Add /ProcessRecursively to include subfolders. Save the command in a .bat file for scheduled or automated runs.

Online vs Desktop

Feature	Online (this page)	Total Outlook Converter (desktop)
File size limit	50 MB	Unlimited
Batch conversion	One file at a time	Entire folders at once
Software install	None required	Windows 7–11
Output options	Standard	Full control
Command-line	No	Yes
Privacy	File uploaded to server	Stays on your computer
Price	Free	From $49.90

Convert OST to HTML Online