A small Office Online change has been made in how you can name / rename a document.
While you have the Office Online doc open in your browser, the title is at the top in the middle of the screen. Double click the title and you can rename it.
I noticed this when I tried to create a new Word Online document directly from the browser. We used to be prompted with a box to name the document. Now, the document opens immediately for editing, with the title of “Document”.