An Oxford hollow is “a flattened paper tube inserted between the spine of a book and its cover to strengthen the spine and allow the book to be opened flat more easily.” But it’s not just like taking an empty toilet-paper tube and flattening it: the tube, if properly made, is constructed in situ. A stout bit of kraft paper is glued to the spine of the book block. Then a piece, almost three spine widths wide is cut and folded. The middle part is glued to the first piece on the spine. This leaves two flaps, A and B, as shown in this diagram (from Cornell Library). B is glued onto the top of A, taking care to keep any glue out of the space below A. Then when you open the book and look down the spine, there’s the tube separating the pages from the binding.

You can see the top of the hollow (and the A/B join) in this photo from Parks Library Preservation.

Jeff Peachey tells when it’s better not to use one in book repair.

If you feel inclined to start checking your book collection to find an Oxford hollow, I suspect you’re going to be out of luck. Most commercial bindings are made with a simple hollow, a single strip of heavy kraft paper glued to the cloth in the spine space between the two boards. Nothing hollow about this at all, but the name has adhered to this degenerate survivor. The most likely place to discover an Oxford hollow in a modern commercial production is in a leather-bound Bible.