I believe a simple workaround is to use a template for a similar setup with 1" square labels and 0.2" cell margins.
The OL5425 should fit, but in my opinion that template is badly made (just a table, where cells will expand to fit content). See my attachment with locked cell size. For optimal utilization of label area I created graphics with a label sized hole in the middle, and wrap to contour. That is what you find on the first page. (First cell's graphics are visible, so you know what I am talking about. The circle cutout graphics are there in the other cells too, but invisible so they won't print.) Second page may be easier to work with but will not utilize label area optimally (cell margins are set to 0.2", so text will use the area of an inscribed square, approximately). Dummy text inserted to show label utilisation.
Please note: The sheet size is "Letter" in my attachment. I can't find any size reference on their pages, so I used the settings from the OL5425 template I mentioned above. If you have A4 sheets (or other), you may need to adjust page margins for good fit, but otherwise I believe it to be useable (haven't tested it on an actual label sheet, though. That will be a task for you

).
Note also that I have saved it as a document (so it does not mess up your editing). Take what you can use from it, delete the rest, and save as template if that's what you prefer to work with.
Apache OO 3.4.1/4.1.2/4.1.3, on Ms Windows 7/10 and Mac OS-X 10.8.5 and 10.11.
LibreOffice 4.3.3.2 on Mepis Antix MX 14