Depends on the font.
And "O-ring" is easier to say (and print) than "rectangle-ring".
"Slipcover" is just as easy to say, and makes a lot more sense.
O-Rings, also known as "O-Cards," have openings on two sides. Slip cases have one opening.
A slipcover has two openings and you slide it over the top (or bottom) of the keepcase. A slipcase has one opening usually (though not always) on the side.