O rings are something you probably played with in public school! We use them to bind books. Silver and gold are standard colours. Many other colours are available but the minimum order requirements are substantial. The main benefit of an O ring style binding is that sheets can be added or removed, O ring binding is different. Customers often want something different.
Picture #1 shows a “wrapped cover” with an eyelet. Picture #2 has a very thick die cut cover. As with all styles of mechanical binding, a wide range of cover materials and thickness can be used. Some O rings are made from plastic and some with a special order that can be “locked permanently!”