Wow, I hadn’t heard about this before. I like that it’s FOSS with AGPL 3. The OnlyOffice screenshots of the spreadsheet application look beautiful (compared to the ugly LibreOffice Calc ones). It says that it works with ODS files (which LibreOffice Calc uses).

Any gotchas to be aware of with OnlyOffice?

https://www.softmaker.com/en/products/softmaker-office is a thing as well, and I prefer it to Only Office. The maker is German.

It's a bit slower performance wise