I've checked OnlyOffice's license. It's server and desktop client use AGPL
v3, so it's barely acceptable.
However, do keep in mind that OnlyOffice is implemented as SaaSS, which has
serious freedom and privacy (and maybe also security) issues, even if the
server and client are free software.
BTW, I only sent OOXML files once in recent years. That was when I was
required to submit my Ph.D. dissertation to my university's library. They
accepted only OOXML files, so I extremely unwillingly edited (one copy of) my
dissertation in the stupidest possible way. For example, the reviewer didn't
know what was a "field", so we had to create TOC using the stupidest method.
