There is no free and open source version of Only Office. It fakes that it is licensed with AGPL, but they have added the following to the license, which in effect completely forbid you to redistribute it. It can be said to be Source Available.
The interactive user interfaces in modified source and object code versions of ONLYOFFICE must display Appropriate Legal Notices, as required under Section 5 of the GNU AGPL version 3.
Pursuant to Section 7 § 3(b) of the GNU AGPL you must retain the original ONLYOFFICE logo in the upper left corner of the user interface when distributing the software.
Pursuant to Section 7 § 3(e) we decline to grant you any rights under trademark law for use of our trademarks.