Is there any option that makes new files be created in ODF formats rather than OOXML formats?
Sorry, there’s no such option. OOXML is the native format for Document server. As a workaround solution you can use ‘Save as’ option in the editor interface or you can send conversion requests to convert desired files to necessary format when editing process is finished: ONLYOFFICE Api Documentation - Conversion API
Thank you for your honest answer.
As Onlyoffice is able to deal with both format, would it be possible to let the user choose what is his/her favorite format and add an option in settings that would allow to do so?
In the file creation scenario, we always use OOXML format: ONLYOFFICE Api Documentation - Opening File
The document editing service downloads the document file from the document storage service using the ID and link provided. At this step the conversion of the file into Office Open XML format is also performed for the document editor better performance and formats compatibility.
In other words, even you uploaded odf file format to Document server, it’s necessary to convert it to OOXML to avoid possible issues during editing process.