Had never even heard of it.
And their website doesn't help at all - https://www.onlyoffice.com/ - At first, I thought it is selling some kind of add-on for Businesses to use with Microsoft Office. Then I clicked "Get it now" and saw a page that gives me options to download a Docker Image, or Cloud Images or VM Images. What the heck? Then I see the "GET OnlyOffice" link in the navigation menu and go to it - the menu pop-up talks about DocSpace, Enterprise, Workspace etc. etc. and asks to sign-in for cloud. Again, what the heck?? Then I go to the PRODUCT link that also pop-ups lot of options - like - Doc editors to integrate into your business platform .... ??? Then I see the "Desktop and Mobile Apps" option in it and click 'For Desktop'. And finally, here I find what I had been searching for - Get free desktop office suite for document editing and collaboration ( https://www.onlyoffice.com/desktop.aspx ).
When a website is designed to be "Enterprisey" - only targetting and catering to a particular audience, and totally ignores everyone else - I am not surprised its user-base is limited. As others pointed out, it doesn't seem like a place where you find free open-source software.
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I downloaded and installed it to try it out, and when I opened it, the first thing I am greeted with, without any warning or information, is a pop-up by the OS informing me that OnlyOffice wants access to my "Chromium Safe Storage" ( https://imgur.com/a/yjA3aJo ). That felt weird and confusing.
But yeah, overall, it does seem more polished than LibreOffice, with a better UI.