How can you change a single page orientation on the online version of Microsoft word?

Sorry, Word Online can't do that by itself.

To be able to separate pages with different orientations, you need to insert a section break between them. As shown in Word Features Comparison: Web vs Desktop, Word Online doesn't have the ability to insert a section break. If you use desktop Word to insert section breaks and change the orientation of one or more pages, Word Online will work with that, but it can't create it by itself.

If you can edit in the desktop version, you can do this, adding section breaks. This is not a feature available in the online version. The changes will then last in the online version.

See Compare Word features on different platforms. (this is an archival copy) current page is Word Features Comparison: Web vs Desktop.

• There are multiple programs, all named Word, and all created and published by Microsoft. This can be confusing when trying to get help.
• They all can edit the same document files that have the same file structure. Some of the programs have no control over some of the document structures.
• But, they are different programs with different features and controls.

As of January, 2024, the most powerful Word program, with the most features, remains the Windows desktop application from Microsoft 365. I do not expect that to change. The perpetual license version Word 2021 is very close as far as features but does not receive new features as added.

The Macintosh desktop application is second with number of features. The things available in the Windows version not yet on the Mac version include Content Controls, Building Blocks, UserForms, and ActiveX. It can use, but cannot create, or modify most Content Controls. AutoText is a Building Block that the Mac can and does use.

The browser version of Word – Word Online – has a Transcribe feature that has recently been added to the Windows desktop version of Microsoft 365’s Word. It also has the ability to Export to PowerPoint (although not that usefully). Otherwise, the browser version has far fewer features and editing in it has been reported to mess up automatic numbering. Chromebooks use a version of this or of the Android mobile app. See How to access Microsoft Office on a Chromebook. This can only be used with documents stored on OneDrive or SharePoint. Differences between using a document in the browser and in the Word desktop application. I would use this for note taking and for quick viewing/printing, but not for editing. Especially not for editing long or complex documents. The ability to mess these up is too high.

The mobile applications (Android/IOS, etc.) vary somewhat but have far fewer features. I would use them for note taking and for quick viewing/printing, but not for editing. Especially not for editing long or complex documents. Primarily these can only be used with documents on OneDrive or SharePoint*. (Usually, you can get at documents on your local device, but it is tricky.)* What you can do in the Microsoft 365 apps on mobile devices with a Microsoft 365 subscription See also Word for Android Phones Help, Word for Android Tablets Help,  Word for iPhone Help, Word for iPad Help.

When seeking help, it is important to give the program that you are having problems with as a part of your question.

Note that the statements about capabilities and usefulness are my opinions, not anything from Microsoft. They are based on my use and on my observations over the years here and in other Word forums. In my opinion, none of the versions is perfect. They can all be useful, recognizing their limitations.