This entire discussion is really useless, mainly because the original topic (this one) was written before the company behind OnlyOffice moved itself to Singapore. Quoting Wikipedia:
In August 2023, OnlyOffice opened a holding company in Singapore and incorporated all the existing branches (those in Latvia, USA, UK, Armenia, Uzbekistan, Serbia) under the one brand. Ascensio System SIA was to become 100% owned by the British-based Ascensio System Ltd, which in its turn was to become 100% owned by OnlyOffice Capital Group Pte. Ltd. The beneficiaries of OnlyOffice Capital Group Pte. Ltd. are listed in the registry of Singapore companies by ACRA.
So, no reason for fearing Russian-controlled bosses.
Besides, as said here, you can just add a decent firewall to block all traffic around OnlyOffice, or just sniff all traffic and see if there is anything suspect - and be 100% sure, lots of people have done that and found ZERO concerns, which is 100% less than what you get if you do the same for MSOffice - or Google Chrome - or Opera Browser - or any Adobe product.
To put it simple: OnlyOffice is by far the best MSOffice replacement available as Open Source (LibreOffice is very good, but doesn’t have the same level of compatibility). Being open source, I wouldn’t really be that worried about it… 