When comparing Emacs Org-mode vs ONLYOFFICE, the Slant community recommends Emacs Org-mode for most people. In the question“What are the best cross-platform writing apps?” Emacs Org-mode is ranked 1st while ONLYOFFICE is ranked 19th. The most important reason people chose Emacs Org-mode is:
This app's flexibility is based on its minimalist approach, giving the user near-infinite freedom.
Specs
Ranked in these QuestionsQuestion Ranking
Pros

Pro Ultimate flexibility
This app's flexibility is based on its minimalist approach, giving the user near-infinite freedom.
Pro Absolutely free
Emacs with Org-mode is free as in beer and free as in speech – that is, it costs nothing and it’s totally open source.

Pro Files are usable anywhere at anytime
Users are not tied to one service provider, program, platform, or database engine.

Pro Incredibly extensible
There are many plug-ins for Org-mode, including Org-habits and Org-notify. If Org-mode lacks some piece of functionality, it is very easy to add it.
Pro Agenda views
Pro Excellent unofficial Android app (orgzly)

Pro Offline support
Pro Efficient features for deadline organization
Pro Supports plaintext spreadsheets
Pro There are a lot of extensions, for exporting to html, bootstrap, js-reveal and much more
Pro Quickly add rich text
Pro Available for Windows, Linux, MacOS, iOS, and Android
ONLYOFFICE offers desktop applications for Linux, Windows, and MacOS that can be connected to the web-based solution for document sharing and collaboration.
Pro High MS Office compatibility
Highly compatible with .docx, .xlsx, .pptx as well as with .odf.
Pro Collaboration capabilities
Pro Can be used as a web app
ONLYOFFICE offers a web-based office suite that can be deployed on your own server using various deployment options, including Docker script and virtual machines with the software pre-installed.
Pro Has a portable version
ONLYOFFICE portable works on almost every Linux distribution.
Pro Advanced work with math equations
Pro Integration with Nextcloud
Easy integration with Nextcloud for collaborative work.
Pro Free and open source
ONLYOFFICE is distributed under AGPL v.3 license with the source code available on GitHub.
Pro Support for add-ons (plugins)
Cons
Con Unintuitive user interface
The key combinations are unintuitive and difficult to remember. This is probably because there are a lot of hidden "modes" depending on where the cursor is. Actions aren't paired with reversing actions like in other todo apps. For example, hitting shift-tab does NOT reverse the effect of hitting tab.


Con Android app isn't very good
There are several user-created apps for Android, but none seem to offer the same level of functionality as other to-do apps.
Con Not really cross platform
Although it is possible to get a lot of it working, no all in one, sync included, out of the box solution is available for mobile devices.
Con By default, a hard-to-read display
The default way of writing an outline or checklist creates a very messy wall of text that's difficult to read with no vertical spacing. You can manually add vertical spacing, but the Org operations don't preserve it. There are pretty-display modes, but you need to remember how to enable them, etc. etc.
Con Web app
It is a text editor that is running in a browser window, how can you trust this? Not to mention that it needs a lot more system resources than native apps.
