There might be any one of a number of confusions here, depending how I read your comment. Or there are none at all. Hard to be sure. But for clarification's sake:
Euro Office is not OnlyOffice. OnlyOffice is not OpenOffice, which is essentially defunct but was the most popular suite that first adopted ODF. OnlyOffice may have been named that way to lure people away from OpenOffice, which was, and is, still in use in some places despite a better non-proprietary option being available. Namely:
LibreOffice is the successor to OpenOffice and uses ODF as its default, so its support is 100%.
OnlyOffice supports* ODF too, but it's by far not the, uh, only one.
* According to their specifications anyway. I haven't used it to be able to confirm how good their support is.